Natural gemstones are creations of nature, waiting to be cut and polished to expose their stunning beauty. Once rare and expensive, many types are now readily available and start at very reasonable prices. They occur as either crystalline materials (example - diamond), amorphous (non-crystalline) materials (example - jade) or organic materials ( example - pearl). There are also polycrystalline gem materials comprising a mass of minute crystals (example - agate, a polycrystalline variety of quartz).
Pearl is the only gemstone rendered complete by nature, all other gemstones normally need to be worked before setting in jewellery. Although, well-formed gem crystals are often used in their natural state.
All gemstones, except diamonds, are cut and polished by craftsmen called lapidaries, diamonds are only cut and polished by diamond cutters who don't cut other gemstones (with rare exceptions). Craftsmen who carve gem material into artefacts or small items for jewellery making may specialise in this form of cutting and polishing only. Jade and green quartz are very popular materials for both.
Gemstones fashioned for mounting in jewellery are either faceted, cut en cabochon or carved.
A facet cut gemstone has many flat surfaces or faces, the Crown facets occupy the upper section of the stone and the pavilion facets the lower section. The thin area between is the girdle, which may be cut round or faceted. Recently facets have been fashioned with convex faces, new techniques are being developed constantly.
The illustration shows a round brilliant cut, an ideal shape for diamonds when cutting from octahedral crystals. It is also a common cut for most other gem types when the rough material gives a better yield when cutting in a round shape. The pavilion on many gemstones is now faceted to give a much greater yield, by using a step cut or Portuguese cut. Both methods bulge out the pavilion between the girdle and the Culet claiming more weight and increasing the price.
When a gemstone has a brilliant cut crown and a Portuguese or step-cut pavilion it is termed a mixed cut. The pavilion of the Portuguese style mixed cut is bloated out by using a lot of small facets, retaining much more material than the brilliant cut. The step-cut pavilion has a series of long facets (in steps from the girdle to the Culet) that also bloats out the pavilion, no single facet spans the girdle to Culet.
Yield is the important factor here, lapidaries will cut and polish rough gem material to gain the highest yield achievable. Cheaper gem types are normally cut to calibrated sizes, as they fit pre-made 'standard sized' settings, but rarely is this done for high-value precious stones. These stones will be cut to standard shapes as close to the gem-rough shape as possible, whilst avoiding cracks and unsightly inclusions. Hence the variety of styles on offer is not so much from choice, but for the sake of yield.
Considering all factors, gemstones are fashioned into standard shapes that include the round, oval, square, teardrop, cushion, marquise, trillion and the octagon (emerald cut). Fancy or non-symmetrical shapes are also faceted and may be modified or carved by a laser device (laser cut). Fancy shapes are often not cut for yield but purely artistic reasons.
A cabochon cut gemstone has only one large domed surface and a slightly curved or flat base, most are cut from opaque, dark or included material.
When cut en cabochon, some gemstone varieties may display chatoyancy, a cat's eye effect seen as a single ray or band of light on the surface of the stone, with a similar shape to a cat's eye. Chrysoberyl, quartz and tourmaline are examples of gemstones that may display chatoyancy. Other varieties of gemstone may display asterism, seen as a four, six or rarely twelve-rayed star effect. Ruby and sapphire are gemstones that may display asterism when en cabochon.
The single or multiple bands of light appear to move across the surface of the gemstone as it is rotated. These effects are made possible by the presence of long thin inclusions running in parallel throughout the gemstone.
The inclusions may be fibrous, needle-like or made of hollow channels from which light will reflect. The gemstones must be cut in the correct orientation for the effect to be seen. The parallel inclusions have to be in-line with the base of the cut stone (perpendicular to the c-axis of the gemstone crystal). Ruby and sapphire display a six-rayed star because the long thin inclusions, known as silk, formed in-line with the three growth directions of the crystal during formation. The growth directions are at 60/120 degrees to each as shown in the illustration above. A twelve-rayed star occurs when two types of inclusion are present, a long thin type together with a small flat type. A corundum crystal that has formed with parallel growth features (twinning) may also display a twelve-rayed star when oriented correctly during cutting.
Carving Gems is an age-old craft using mainly polycrystalline materials. Large pieces are available for carving into ornamental artefacts, they include Jade, lapis lazuli, quartz varieties, (example - chalcedony) malachite and other mainly polycrystalline gem types. Carvings for sale in shops and markets are often not made of the material they appear to be or are described as. Green quartz is often sold as jade (nephrite jade) and is so remarkably similar that it can be sold right next to genuine jade. The structure of polycrystalline materials also allows them to be easily dyed so they can be colour enhanced or dyed a different colour to simulate a different material.
Most gem types are also carved for setting in jewellery, precious opal is becoming more popular for this. A much greater yield is achieved when compared to cutting en cabochon. It also allows for a continuous colour pattern that may be destroyed when rounding a stone.
Intricately carved artefacts have been produced for thousands of years from fairly basic tools. Modern lapidary techniques now employ motorized and manual diamond tools that allow creative designs in much less time, limited only by imagination. Diamond is used as it is the hardest known natural material, so how can a diamond be cut and polished? Diamond has directional hardness, it can be cleaved (split) through perfect planes then cut and polished by other diamonds held in the correct orientation. Lasers are also used to cut diamonds, this reduces the chance of the stone shattering due to internal pressure or when challenging flaws are present.
Gemstone enhancements are not a new idea, they were first documented by the Roman historian and naturalist Gaius Plinius Secundus (AD23-79). Known as Pliny the Elder, he wrote many books on natural history and his study of gemstones was extensive. He was concerned that treated gemstones were being portrayed as more valuable gemstones and used in a lucrative trade of forgery. The treatments listed in his writings include oiling, dyeing and heat treatment.
It is well known that the majority of the gemstones on today's market are treated to enhance their appearance in some way. Without treatments, there would be few visually acceptable gemstones available to purchase. The most common method used is heat treatment where high temperatures are applied to either lighten, intensify or modify the colour of gemstones. Also to improve clarity and remove unwanted colour tints. Rubies, sapphires and aquamarines are examples of gemstones that are commonly heat treated, the process takes many hours in custom-made high-temperature ovens. Some gem materials are irradiated (and often heated too) to produce colours that rarely occur in nature, blue topaz is one example.
In another heating process called Surface Diffusion or Lattice Diffusion, pale stones are coated in chemicals and once heated, a thin layer of colour is diffused into the surface of the stone. The colour transformation is remarkable, but if the stone is re-polished with abrasives it will revert back to its former colour. Chipped stones will also reveal the original colour. Beryllium diffusion is a fairly new process that can also penetrate deeper into the gemstone and even completely change the colour of some stones.
Other treatments include fracture filling, dying, bleaching, foiling (coloured foil behind transparent gemstones) and the application of high tech coatings.
Some gemstone varieties are not commonly treated (if at all) they include tourmaline, spinel, peridot, varieties of garnet and precious opal (not including the Matrix/granular type).
Composite stones are made by glueing sections of different gem material and/or non-gem materials together. The crown of a faceted gem may be of a precious material, whereas the pavilion may be of glass, quartz or even a synthetic form of the actual precious material itself. Some are even made with the same gem material for both the crown and pavilion of a faceted gemstone.
Doublets are composites comprised of 2 layers of material, whereas triplets have three layers, with the precious material sandwiched between cheaper materials. Opal is commonly cut into doublets and triplets to utilise thinner (but still very precious) material. Doublets are made by glueing the opal to a base of black potch (common opal), obsidian (volcanic glass) or other dark material. Triplets are made by glueing a very thin opal layer to a base material and then applying a protective quartz cap. Both opal composite types used to be fairly cheap, but prime examples made from rare material now fetch surprisingly high prices.
Gemstone enhancement and alteration is a vastly complex subject and a highly guarded secretive process in many gem factories in the far east. Where new techniques are being invented and then further developed on a continual basis. The industrial rewards are immense, where low-quality precious gem ducklings are be turned into magnificent gem swans to feed a very hungry gem market.
Terms used when describing gemstones
The characteristics of colour are described separately as:
Hue - the actual colour seen, known as the primary colour such as red and blue. When the colour is between two colours, then both colours are mentioned, for example - yellowish-green or bluish-purple.
Saturation - the amount of colour present. Gemstones are often heat-treated or irradiated to increase the colour saturation.
Tone - the lightness or darkness of the colour present, also described as the Depth of Colour.
Colour Purity - the purity of a colour is influenced by the presence of grey or brown. When grey or brown is absent then the colour is of high purity; often termed as vivid or intense. Colours of high purity can be seen in the light dispersed through a prism or the facets of a white diamond.
Some gemstones show more than one colour, for example:
Particoloured stones - show two separate colours in one stone, an example is Ametrine.
Colour change stones - are one colour in natural light and another in artificial light, an example is Alexandrite.
Multicoloured stones - show at least three colours, an example is Multicoloured Opal.
Gemstones possess varying degrees of transparency known as diaphaneity, described as either:
Transparent - light passes through the gemstone and objects can be clearly seen through the stone.
Translucent - light passes through, but objects may be seen merely as shapes or shadows.
Opaque - no light passes through the gemstone.
The amount and quality of light reflecting from the surface of a gemstone is known for its lustre.
For example, vitreous is a "glass-like lustre" and adamantine is the 'very high lustre' seen on the surface of a polished diamond. The lustre of gemstones is described in terms that are easy to relate to, for example metallic, resinous, pearly, silky, greasy, waxy and dull.
It is usually found (with exceptions) that the harder the gemstone material, the brighter the lustre.
The quality of the surface polish is important, as a poor polish will display a poor lustre.
Brilliance is the amount of light reflected from the crown facets of a facet-cut gemstone. It is a combination of the light reflected directly from the facet surfaces and the light that has entered the stones and is reflected back out. Two factors critical to achieving maximum brilliance are the angle of the pavilion facets and the degree of polish on the facet surface (lustre). The table facet of a diamond is cut to various sizes depending on the desire for more brilliance and less fire (dispersion - see below), where a larger table is used. Or more fire and less brilliance, where a smaller table is used. A good balance is obtained by cutting to ideal proportions.
The pavilion facet angles are not only critical in determining the amount of brilliance if cut too shallow a 'window' is created where light escapes through the stone. Windows are unattractive as the background can be seen clearly through the stone and colour is lost in coloured gemstones. Also, cutting the pavilion too deep will result in dark areas in the stone, termed extinction. Brilliance is lost in both unfortunate situations.
Dispersion is the splitting of white light into its spectral colours, as seen with light transmitted through a prism. It is also called fire when describing the spectral colours coming from gemstones such as diamond, which has a high dispersion value. Diamonds may be cut to increase dispersion at the sacrifice of brilliance or vice versa as described above.
The fire in opal is caused by the diffraction of light through its unique hydrous silica structure.
The amount of dispersion in gemstones can be given in figures but is generally described simply as low, medium and high.
Dichroism - is a gemstone property, where two colours or shades of colour are seen when a gemstone is viewed from different directions.
Trichroism - is a gemstone property, where three colours or shades of colour are seen when a gemstone is viewed from different directions.
The ability of a crystalline gem material to split or crack along a flat plane within its crystalline structure. Cleavage can be perfect, imperfect, poor or it does not occur.
The Mohs' scale of hardness determines which stones are harder than others. Harder stones have the ability to scratch softer stones and are positioned higher on the scale. The scale is a 'comparison scale' only and does not determine absolute hardness. Diamond is the hardest at 10 on the scale and talc is the softest at 1.
The resistance to damage or critical fracture when confronted by a hard physical knock or pressure. Cleavage is a factor when considering toughness. Jade is a good example of a strong gemstone.
The durability of a stone takes into account its hardness, toughness and ability to resist chemical erosion. All factors must be considered; a hard stone, for example, may be brittle and suffer damage to the facet edges.
Some gemstones are susceptible to sudden changes in temperature, termed thermal shock. This may cause crazing (surface cracking) or serious fractures. Immersing hands in hot water whilst wearing rings set with gemstones or moving outside from warm to freezing conditions are examples of sudden or severe temperature change.
A gemstone is called a variety when it belongs to a species, e.g. emerald and aquamarine are both varieties of the species beryl. Varieties must have significant differences to be called varieties, such as a colour variation or special optical effect.
To be a species there must be at least two varieties, for example, corundum is a species with the varieties sapphire and ruby. Beryl is another example of a species with many varieties including emerald, aquamarine and morganite.