Textures in Gemstones

Gemstones may be defined as a naturally occurring substance (includes minerals, group of minerals, mineralloids or even organic materials) that are rare, beautiful and durable. Many times, specific names are used for gemstones belonging to the same mineral species but different in their outer look. For example the mineral species corundum, that is found in nature and formed through natural processes, crystallises in hexagonal system with a chemical composition of essentially aluminium oxide (Al₂O₃). Most corundum grains do not bear the properties to be of gem variety. But if corundum contains trace quantities of chromium replacing aluminium in the crystal lattice, it renders to a stunning red colour to the crystal; known as ruby, a coveted gem variety of corundum. Its cost increases depending upon the saturation of the colour, clarity and its transparency. Apart from ruby, the most precious gem variety of corundum is blue sapphire which contains insignificant quantities of iron and titanium in the lattice imparting the rich blue colour. Similarly, in case of the mineral species beryl (silicate of beryllium and aluminium), the vivid green gem variety is referred to as emerald where aluminium ions (Al₃₊) are substituted by some chromium ions (Cr3+).The other gem varieties of beryl are aquamarine (blue-green), heliodor (yellow), morganite (light pink), goshenite (colourless/ white). Certain gem materials, composed of more than one mineral, like lapis lazuli (consisting of the principal mineral, deep blue ‘lazurite’ with other accompanying minerals), ‘jadeite’ (with associated rock-forming minerals; popularly known as ‘jade’), aventurine quartz/feldspar (e.g. green fuchsite mica-bearing quartzite /feldspar with the inclusions of stunning orange-red flakes of haematite-bearing ‘sunstone’), etc are categorised under “aggregate of minerals” or rocks. Apart from the gem minerals produced by inorganic processes, gem materials may also be formed from organic activities of some organisms or by their fossilised remains. According to the CIBJO Retailers’ Reference Guide 2020, this entire category is designated as “biogenic gem materials”. Within this, one group that is essentially associated with biogenic organic matter is named as “organic gem materials”. This group consists of ivory, tortoise shell etc. Pearl, mother of pearl, abalone shells and precious coral are biomineralised calcium carbonate and are considered as simply “biogenic gems”. Jet and amber are better considered as fossils.

Although, the term “gemstone” refers to only materials that are found in nature, owing to the high demand and less supply in market, synthetic materials akin to certain important gemstones are produced in certain high end scientific laboratories that are considered to be completely or partially “man-made”. The other laboratory method involved with gemstones are “treatments or enhancements” of natural poor quality materials.

Without a gemmologist’s correct identification and certification, a potential buyer or even traders can very easily be deceived by replacement of a natural stone with a “counterfeit” stone. A synthetic stone of identical chemical, physical and optical properties; a natural stone of different species but some identical features like colour; a synthetic stone of different species but identical features; stones treated for colour and clarity; stones whose features has been modified deliberately; composite stones; all can be used as counterfeits for natural gemstones in the gem market.

On the basis of origin, gem materials can be divided into the following categories: Natural gem materials and artificial products.

Gem materials, including those of organic and biogenic origin, are rare, beautiful and durable; and have been formed by natural processes without any human interference. According to The Gemstone Book by CIBJO, it encompasses all natural minerals (aquamarine, diamond, emerald, garnet, opal, sapphire etc), natural glasses (obsidian, maskelynite, tektites etc), rocks or mineral aggregates (lapis lazuli, turquoise, malachite- azurite etc), organic gem materials (natural pearl, precious coral etc), biogenic gem materials (tortoise shell, rhino horns etc) and fossils (jet, amber etc) that are used in jewellery or objects d’art with the exception of metals.

Other than the natural gem materials described above, two more different variety of natural gem material exist in the gem market. The first category is Treated Gemstones, in which gemstones are treated by various laboratory processes to change or enhance their appearance and or durability. Natural minerals may have flaws either in color or inclusions which render them unsuitable to be treated as gemstones. However, over time, man has developed a variety of processes of treatments of these inferior natural non-gem materials to upgrade their beauty and clarity so that they may be easily marketed. For example, a fractured ruby (red corundum) is normally unsuitable to be treated as a gemstone as the air within the fractures cause refraction of light thereby decreasing clarity of the grain. However if the fractures are filled with glass of comparable refractive index, this problem can be somewhat eliminated. Similarly conversion of dull, off colored and cloudy sapphire (Geuda sapphire) into a beautiful transparent blue sapphire by heating in an appropriate environment (oxidising/reducing) is a classic example of enhanced or treated gemstone. CIBJO has also recognized a separate category as ‘Treated Gemstones’ under Gemstones, where inferior gemstones have been treated by various processes like heating, diffusion, irradiation, filling, coating or other artificial processes to change their appearance and or durability.

Another variety of natural gemstone exist which are of lower market value and completely different mineral with different chemical composition, but looks similar to the high value natural ones, for example a low value red spinel can be confused as high value ruby or a blue kyanite can be mistaken as a blue sapphire. Such gemstones are called Natural Simulants.

Synthetics: Artificial products having essentially the same chemical composition, physical properties and structure, as that of their naturally occurring counterparts. A synthetic ruby has similar composition and crystal structure as that of a natural ruby, but the former is laboratory made while the latter is a product of natural processes in the earth crust. Hence their internal inclusion patterns would be completely different, thus helping in their identification.

Imitations: Artificial products that imitate the appearance of natural materials without having their chemical composition and/or their physical properties or their structure. A green glass can be a simulated counterpart for Emerald, Cubic zirconia /American diamond is an imitation of natural diamonds.

Study of gems and gemstones involve various non-destructive studies which includes the conventional ones (followed in Gemmology Laboratory of GSI, NCEGR, Kolkata) like preliminary identification of a gemstone by its colour and studying its signature in diffraction grating type spectroscope, determination of specific gravity, refractive index, checking for fluorescence and/or phosphorescence and study of the internal features (inclusions and flaws) using a loupe followed by a microscope. Advanced studies done with Electron Microprobe (EPMA), Scanning Electron Microscope with Energy dispersive X-Ray Spectrometer (SEM-EDAX), Laser Raman Spectroscopy, Fourier Transform Infra-Red Spectroscopy (FTIR) etc. Only if a given specimen of rough stone is big enough, destructive tests like chemical analysis can be taken into consideration with the permission of the owner of the stone. However while testing a cut and polished gemstone, it should be noted that the efficiency of a gemmologist lies in identifying a stone without subjecting it to any kind of destruction.

One of the most important and characteristic study for authentication of origin of gem minerals (natural, synthetic or treated) is observation of various internal features designated as “flaws and inclusions” (solid, liquid and gas) under high magnification of microscope. Natural simulants can be separated from their natural counterparts using other physical properties like specific gravity and refractive index. Besides authentication of their origin, inclusions may often play an important role in increasing the aesthetic beauty of gemstones by various special optical phenomena like asterism, chatoyancy, colour zoning to mention a few.

Since gemstones are mostly distinct minerals, their textures principally refer to various internal features; such as characteristic “inclusions or flaws” for the given mineral/stone observable under microscope.

The most common inclusions observed under microscope are as follows:

During growth of a mineral in a natural environment, it can host or entrap already formed pre-existing solid mineral grains (protogenetic inclusion) like rutile grains in quartz or corundum crystals. Sometimes inclusions are formed at the same time as the host mineral. These inclusions are therefore considered to be of the same age as the host crystal (syngenetic inclusion). These inclusions can be solids (of the same or different composition: spinel crystalline inclusions within spinel gemstone and pyrite crystals in Columbian emerald respectively), fluids (water or carbon dioxide or hydrocarbon: fluid inclusion in topaz, beryl, quartz etc), or gases (carbon dioxide, water vapour, etc) or compound gases which are combinations of any of the three forms of matter i.e. gas bubbles associated with liquid and/or solid inclusions (Fig. 13.20). Some of the inclusions are formed after the host crystal was formed. These inclusions are usually either formed by the process of exsolution or from the recrystallization of remnant fluid along some healing fracture in a host mineral. They may also be liquid, solid, or gaseous (Fig.13.17).

1.Mineral grains: Solid mineral grains, whether of the same composition as that of the host mineral or of some completely different mineral, occurring within a gemstone are named as “mineral grains”. At times radioactive mineral grains can get trapped within gemstones, and the halo (zone of alpha particle induced damage of lattice of host crystal) created by its radiation is visible under microscope.

2.Feathers: This term is a trade name used for secondary fluid inclusion trails along healed fractures within gemstones.Feathers can be both syngenetic and epigenetic with respect to the growth of a crystal. During the growth of a crystal, it may be subjected to stress and/or strain which cause the formation of fractures or fissures in gemstones. Presence of any primary inclusion of mineral grains may also lead to local stress field around the inclusion and resultant fractures. These fissures, immediately after their formation goes through healing. The fluid that permeates into these fissures, forms trails of fluid inclusions along the healed surfaces. This process may continue during the entire process of crystal formation. Such feathers are syngenetic. If a fracture is developed after the formation of a crystal and is healed eventually, then the resultant feather will be epigenetic in origin.

3.Fluid inclusions: During the process of formation of a crystal or after its fracturing, some fluids from the surrounding environment may get trapped inside the crystal as fluid inclusions. Similar to feathers, they too can be syngenetic or epigenetic in origin.

4.Silk: This is name given to the microscopically fine needle shaped mineral inclusions or exsolution lamellae within natural gemstones like ruby, sapphire and garnet. These are arrayed in groups along particular crystallographic orientations of the host mineral. The groups intersect with each other forming subsurface reflections to produce a bright-reflective sheen resembling that of silk fabric. The most common mineral to form silk is the shimmering slender rutile needles, and in some cases tiny platy and fibrous particles of minerals such as haematite/magnetite, relict asbestos, and micaceous minerals may also occur. Well preserved silk in a gemstone is a direct confirmation of its natural origin.

Other than the above mentioned ones, natural minerals often contain some patches of fissures, growth lines, colour zoning etc formed during the formation of the minerals.

5.Fissures:Fissures are small, narrow gaps or planes of discontinuity occurring within gemstones. The mechanical stress leading to the formation of fissures can be both natural and man-made. Hence fissures are considered only as weaknesses in gemstones and do not provide any indication about their origin. Fissures are often utilised during treatments for filling the non-gem minerals with coloured glasses or dyes to produce “glass-filled or dyed gemstones”.

6.Growth Lines and Colour Zoning:Growth lines in gemstones are described as subtle lines of heterogeneities, caused by slight changes in optical properties of the growing crystal, due to variations in growing conditions, crystal structure defects or other causes during the process of growth of the particular crystal of gem mineral. Colour zoning in gemstones are formed due to uneven incorporation of certain colour inducing trace elements during the formation of a particular crystal of gem mineral. Straight or angular growth lines and colour zoning are a confirmation for the natural origin of a gemstone.

7.Twinning and Parting planes:A few of the gemstones such as corundum and feldspars are characterised by twinning and parting planes. Growth twins and deformation/gliding twins are the two important twinning found in minerals. The former is developed at time of growth of the mineral, while latter is post-growth due to stresses induced during deformation. The growth twins may be simple twins with two individual join together at certain crystallographic direction or polysynthetic/multiple twinning (several individual members join together) at certain crystallographic direction. Many corundums occurring in metamorphic terrain exhibit polysynthetic twinning in three directions parallel to the rhombohedral faces due to their involvement in some post-growth deformation. These twin planes resemble cleavages.

Being the hardest natural mineral and due to its origin in mantle, diamonds often show some special internal features that are unique to diamond itself and not seen in any other minerals.

1.Trigons: Trigons are triangular surface resorption features that are formed after the growth process of a diamond, due to partial removal of crystalline diamond (also known as dissolution) that occurred when the diamonds continue to remain in the mantle. When observed under microscope, trigons are triangular etchings or pits on the surface of a rough diamond that are oriented 180 degrees from the octahedron faces. As trigons are very difficult to cut and polish, they may sometimes be retained as such. In fact, the presence of the retained trigon confirms the natural entity of the stone.

2.Tetragons: Similar to trigons, tetragons are also resorption features seen in diamonds that were originally cubical in shape in their original rough, uncut state. The square outline of the tetragon reflects the four fold symmetry. The corners of the tetragon generally points to the edges of the underlying crystal face in negative orientation, i.e., corner pointing to edges and edges to corners. In some polished diamonds, the tetragons may be retained indicating its initial cubical form.

In the gemstone market, the demand is always greater than the supply. Availability of the best or even good quality gemstones is always less in comparison to the ever-increasing demand. In this scenario of unmatched demand and supply, treatments or enhancements of inferior quality gemstones come to play an important role. From any gemstone mine, the percentage of gem quality minerals obtained is limited as compared to the large quantity of the same mineral of poorer quality. This non-gem variety has little or almost no market value. Gemstone researchers have developed methods to enhance one or more than one of the properties like colour, clarity, weight etc. for this huge quantity of low grade material and thus creating market for these stones. The effects of treatments sometimes leave behind their characteristic internal features which help in their identification.

Some common features within treated gemstones as observed under microscope are:

1.Glass filling in corundum: The gem variety of corundum (ruby, blue sapphire and yellow sapphire) extracted from any mine, is accompanied by poor coloured to colourless, highly fractured corundum with very less clarity. In order to convert them into marketable products, they are filled with appropriate coloured glasses. The lead glass (for ruby and yellow sapphire) or cobalt glass (for blue sapphire) having similar specific gravity that of corundum is used to fill the fractures at a high temperature of around 900 degree Celsious to enhance the clarity. Under microscope such glass filled fractures will indicate flashes of colours owing to their difference in refractive index with the host mineral, i.e. corundum. During this process of high heating the ‘silk’ (exsolved acicular needles of rutile), if present within the crystal, condenses to form droplets resulting in “diffusion of silk”.

2.Dyeing of Minerals: Quartz is a common mineral which is often dyed in order to imitate other gemstones. The process of dyeing quartz is called quench crackling. In this process the gemstone is first heated and then subjected to quenching in a cold, liquid solution—such as water. Due to quick and sudden cooling, fractures are developed within the gemstone that radiate throughout. Since the fractures produced are surface-reaching, the quartz is then subjected to additional immersion in a dye solution, allowing the fractures to be filled with coloured liquid. In order to produce an imitation of emerald, green dye is used. When these dyed gemstones are observed under microscope, it is found that the colour of the dye is concentrated only along the fractures in the gemstone, whereas the rest of the gemstone remains colourless. In view of the fact that such stones are highly fractured, so the presence of colour concentrations within the closely spaced fractures imparts an over-all green appearance to the gemstone.

3.Laser drilling: Presence of visible impurities or inclusions is considered as a negative quality in diamond as it reduces the clarity of a particular crystal. In order to reduce the visibility of such impurities, a special treatment of laser drilling is done. In this treatment a minute tunnel is drilled within diamond in order to remove the visible impurity.

4.Heating/ Heat Treatment: The exposure of a gem to high temperatures for the purpose of altering its colour and/or clarity is called heat treatment. The most commonly heated gem minerals are:

• Amethyst: Heated to remove the unwanted brownish stains and in some cases, amethyst is heated to form citrine, the yellow variety of quartz that is often passed on as yellow sapphire or yellow topaz.

• Aquamarine: Heated to remove the greenish colour component thus creating a bluer gem.

• Ruby: Heating to remove the purplish undertone thus intensifying the pure red colour.

• Sapphire: Heated to intensify or even “create” the blue colouration

• Tanzanite: All the gem tanzanites available in the gem market are brown coloured zoisite that has been heated to remove the brown component and to produce a stronger purplish blue colour.

Since very early stage of Gemmology, attempts have been taken continuously to imitate the beauty of the gemstones available in nature with man-made processes. With the advancement of science and technology various processes had been invented to imitate the natural gemstones with synthetic ones. While the creation of synthetic gemstones has helped the industrial use of gemstones to a great extent, but it has also adversely affected the market and valuation of natural gemstones. Hence it is of utmost importance to separate the synthetic gemstones from their natural counterparts. Fortunately, the various processes by which synthetic gemstones are produced leaves behind some signature features within the gemstone that are easily identifiable under microscope thus helping in the process of separation. Some of the common inclusions observed in synthetic gemstones are as follows:

1.Gas bubbles: This feature is commonly observed in synthetic corundums produced by Verneuil process. Also known as Flame fusion process, this is the least expensive and the most common way of forming synthetic gemstones. In this process the raw materials required for the formation of a particular mineral are dropped through a high-temperature flame when it melts and falls on a rotating pedestal thus forming a synthetic crystal. This process of melting leads to the formation of gas bubbles within the crystal, making it a signature feature for the synthetic crystals formed by Verneuil method.

2.Curved colour lines/bands: This is also another signature feature created in the Verneuil process. When the raw materials are dropped from above, at first a tiny molten droplet is formed and the consequent molten drops are deposited over the first droplet. Thus the growth lines are formed in a semi circular layer towards the top, which are retained within the gemstones even after cutting and polishing, thus helping in their identification.

3.Flux fingerprints: Flux is a solid material that, when melted, dissolves other materials (raw materials for gem mineral formation) in the same way that water dissolves sugar. As the dissolved chemical solution gradually cools, freshly crystallising gem material is added to the system. However, during their formation, the solidified crystals often incorporate within them some of the molten flux as well, which in turn appear like natural feathers. These are referred to as ‘flux fingerprints’.

4.Honeycomb Texture: In several flux-fusion created synthetic corundum, the healing fractures have a distinctive, continuous, three-dimensional inter-connected cellular structures that are referred to as Honeycomb texture.

5.Wispy Veils: This type of inclusion is common in Flux-grown Emeralds. They are high in relief and have a ribbon like or wispy veil like appearance.

6.Chevron patterns: This particular feature is seen in Synthetic emeralds produced by hydrothermal process. This process required heat and pressure and it imitates the conditions prevailing deep inside the earth, where gem minerals are formed. Raw materials are dissolved in an aqueous and synthetic emerald crystals are formed as the solution cools. Synthetic emeralds formed by this method show a typical ‘chevron pattern’. When observed under microscope, a V-shaped pattern is found to be repetitive throughout the crystal and all of them point to the same direction.

7.Nail Head Inclusions: Another signature inclusion found in synthetic hydrothermal emeralds, and occasionally in flux grown synthetic emeralds and synthetic quartz is the spike on ‘nail-head’ inclusions. The “spike” of the “nail head” inclusions are wedge shaped two-phase (liquid and gaseous) fluid inclusions that are capped by some crystals (i.e the nail-head), thus acting as growth obstacle. Relatively disturbed growth is the main cause for the formation of nail-head spicules. During the growth of the host synthetic crystal, a small crystal or a platelet is deposited near the seed plate. In synthetic emeralds, this platelet is often phenakite, beryl, chrysoberyl or even may be gold from the crucible. These platelets form the “nail-head”. As the host crystal continues to grow past the platelet, a tapered void is created which traps the hydrothermal growth medium. On cooling, it becomes two phase, consisting of a liquid and a gas bubble.

Among the imitations that are created in labs for various gemstones, cubic zirconia (CZ) is one of the most commonly encountered one. When colourless / white, it forms an excellent imitation for diamond, whereas the colored ones can be used to imitate many gemstones, like red for ruby, yellow and blue for yellow and blue sapphires respectively. Sometimes the coloured ones are also passed on as imitations of coloured diamonds. Other than cubic zirconia, the other common imitation is glass. However, CZ and other diamond simulants can be easily eliminated using ‘thermal conductivity meter’. Various coloured glasses are used to imitate a variety of gemstones, including opaque red glass for red coral and chatoyant glasses (made with bundles of fibrous inclusions placed parallel to each other) for stones with cat’s eye effect.

When observed under microscope, glass imitations often show signature features, thus facilitating their identification. A glass cat’s eye, when seen perpendicular to the parallel fibrous inclusions causing the cat’s eye effect, shows the cross-section of the bundles of the fibrous inclusion, resembling the texture of the skin of a lizard.

Another signature observed in artificial glass is when glass is fused and retained at high temperature for too long; nucleation of certain crystalline materials may take place. The presence of foreign residue such as dust on the surface of the glass or inside the kiln prior to firing can provide nucleation points where crystals can propagate easily. The chemical compositions of some types of glass can make them more vulnerable to devitrification than others. The green coloured glass, which is often passed on as imitation of natural emerald shows exclusive signs of devitrification.

In case of natural gemstones, inclusions are often referred to as “flaws”. Since inclusions are a natural obstacle in the pathway of light passing through a faceted gemstone, the presence of inclusions may often reduce the clarity and hence the beauty of the gemstone. Gemstones with numerous inclusions are often easier to find than clear one with less inclusions. Hence, the value of gemstones with abundant inclusions and lesser beauty are always less than the more clear and flawless ones. Presence of inclusions visible under 10 x magnifications may reduce the overall clarity and therefore the price of the gemstone.

However, for gemmologists, inclusions act as a blessing as they often bear signatures about the origin (natural, treated or synthetic) of the gemstone. Other than providing information to the gemmologists, certain types of inclusions are capable of imparting aesthetic beauties to certain gemstones. Some of the common positive aesthetic effects of inclusions are as follows:

• Inclusions are often known to provide colours in some allochromatic minerals.Common agate (shows bands of colours due to various inclusions),jasper (coloured red, yellow or brown by the oxides of iron) and chalcedonies (with wide variety of colouring inclusions); rose quartz (fine dumortierite needles); aventurine quartz (tiny inclusions of green fuchsite mica); bloodstone (opaque green coloured microcrystalline quartz with red-jasper-spots due to iron oxides); moss agate(inclusions of chlorite forming greenish dendritic patterns); rutilated quartz [sagenite (Venus-hair stone etc) - rutile needles in quartz] are a few names worth mentioning.

• Inclusions at times may impart a special optical phenomenon to a given gemstone thus enhancing its beauty.The common ones are: Chatoyancy (parallel sets of elongated needle like fibrous inclusions imparting cat’s eye effect in gemstones:Fig. 13.62); Asterism (regularly arranged fibrous/platy inclusions meet each other at an angle and shows a star effect when a single source of light falls on them from perpendicular direction:Fig. 13.63); opalescence (play of colours observed in gem variety opal, both white and black, as light is refracted from the microscopic silica spheres: Fig. 13.64 a and b); Adularescence (milky white to bluish white glow seen in moonstone caused by scattering of light:Fig. 13.65) etc.

CIBJO-Official-Gemstone-Book dated 22-12-22

CIBJO- Retailers’ Reference Guide

Diamonds in Nature: A Guide to Rough diamonds: Book by Michelle C. Tappert and Ralf Tappert

http://www.gia.edu

Gems and Gem Industry in India: Book by R.V. Karanth

Flux induced fingerprint patterns in synthetic ruby: An update: K. Schmetzer and F.J.Schupp, Gems and Gemmology, Spring, 1994

A Study of Nail head Spicule Inclusions in Natural Gemstones: Gagan Choudhary and Chaman Golecha, Gems and Gemmology, Fall, 2004