Gemstones
Authored by Cynthia B. Reuschel, RGA and contributed by Search4gems.
Education | Diamonds | Pearls | Opals | Precious Metals | Jewellery Care |
||||||||||||||||||||
It's all about colour! |
||||||||||||||||||||
| Colour should be the first thing you think about when shopping for gemstones. There are several methods of describing gemstone colour, but they all essentially describe the same properties: hue, tone and saturation. Hue describes the primary colour of a stone and any other visible hues which modify the primary hue. A "red" stone isn't just a red stone. It can be purplish-red or orangish-red or a host of combinations in between. Tone refers to the lightness or darkness of the hue and is generally expressed as very light, light, medium-light, medium, medium-dark or dark. Saturation describes how much of the primary hue a stone contains. Stones described as having strong or vivid saturation have very little brown or gray interfering with the primary hue and generally are considered the most desirable. |
||||||||||||||||||||
Clarity |
||||||||||||||||||||
| Most gemstones are colourless in their purest form. However, most of the time, the gemstone's crystal will contain impurities which cause it to have colour, and it most likely will have some type of inclusions as well. Some gemstone varieties are by their nature more included than others. It is rare that for a coloured stone not to have inclusions, and, in some cases, inclusions are desirable, such as the horsetail inclusion which is often present in demantoid garnet of Russian origin. The Gemological Institute of America (GIA) grading scale divides coloured gemstones into three types based on the amount of inclusions usually present in them. Clarity grades are based on the number, type and size of inclusions found in a stone, their location within the stone, and their overall effect on the beauty and durability of the stone. The GIA has a grading scale for each of the three gemstone types which addresses the number and type of inclusions normally found in that type. For example, a Type 3 VS gemstone will have more inclusions than a Type 1 VS stone. |
||||||||||||||||||||
GIA Coloured Stone Clarity Grading Charts for Type 1, 2 and 3 Gemstones |
||||||||||||||||||||
|
||||||||||||||||||||
|
||||||||||||||||||||
|
||||||||||||||||||||
Shape and Cut |
||||||||||||||||||||
| The terms shape and cut are not interchangable. Shape refers to a gemstone's overall outline. It can be round, square, oval, pear, rectangular octagonal, triangular or trilliant, to name a few. Cut refers to the number and arrangement of the facets. Each shape can be cut many different ways. Some popular cuts are known as brilliant (for diamonds), Portugese, step, radiant and mixed but there are many, many more. There are a few terms, however, that describe a shape and a cut, such as emerald cut or princess cut. There are also patented cuts, such as the Royal Asscher, which is a square variation of the emerald cut. A bad cut can ruin a beautiful gemstone. Most colored stones on the retail market are native cut or cut in commercial cutting houses. Native cut stones are cut in the country of origin by cutters using native style equipment. The stones are often "windowed" or are cut for maximum weight, which saves money, but doesn't always do justice to the stone. Commercially cut stones are generally cut to calibrated sizes and can vary in quality and consistency. Custom cut stones can take many forms and are limited only by the imagination of the cutter. The cost of a custom cut stone can be several times that of the same native cut stone, but there really is no comparison and the custom cut is worth the extra cost. |
||||||||||||||||||||
An Introduction to Gemstone Treatments |
||||||||||||||||||||
| Gemstones have been valued throughout the ages for their beauty and rarity. Advancements in mining technology, as well as global marketing, have created more demand for coloured stones, and a variety of enhancement processes have been developed which ultimately have made gemstones available to many who would never be able to own them otherwise.
Several methods of gemstone enhancement are common in the market today, and, with proper disclosure, are generally accepted within the trade because they have been demonstrated to be stable and permanent. Untreated stones are available from many Search4Gems sellers and elsewhere, but the vast majority of commercially sold gemstones have been treated by one or more of the following methods. Very few coloured gemstones have not been successfully treated to enhance colour or clarity and new techniques for enhancing coloured gemstones are continually being developed. These may be difficult, or in rare cases, impossible to detect, even for the most sophisticated laboratory. For more complete information on coloured gemstone enhancements, see the American Gem Trade Association's Gemstone Information Manual. BleachingThe use of heat, light and/or other agents to lighten or remove a gemstone’s colour. This is sometimes used in pearl processing. The bleaching is permanent and stable, but pearls by their nature require special care. CoatingThe use of such surface enhancements as lacquering, enameling, inking, foiling or sputtering of films to improve appearance, provide color or add other special effects. All "mystic" type topaz is coated and requires special care because the coating may easily be scratched. DyeingThe introduction of coloring matter into a gemstone to give it new color, intensify present color or improve color uniformity. This is used in many materials, including jade, chalcedony, agate, pearl and lapis lazuli. Certain dyed materials may fade in light or heat, and extra special care should be taken to avoid chemicals, cosmetics and ultrasonic cleaning. DiffusionThe use of chemicals in conjunction with high temperature to produce artificial color change and/or asterism-producing inclusions. It is a violation of the FTC Guides to fail to disclose diffusion on gemstones in advertising, promotional literature or commercial documents. Ruby and sapphire are commonly diffusion treated to improve or change color and to produce a "star." Red and green andesine-labradorite not from Oregon has been diffused to produce its colour. FillingThe filling of surface-breaking cavities or fissures with colorless glass, plastic, solidified borax or similar substances. This process may improve durability, appearance and/or add weight. This process has become widespread in the market, particularly in ruby and sapphire. Heat TreatmentThe use of heat to effect desired alteration of color, clarity and/or phenomena. The application of heat to enhance the colour and/or clarity of gemstones has been a common practice around the globe for centuries. It is part of the standard polishing and finishing process for many coloured gemstones. As such, it is accepted by the jewellery industry and the American Gem Trade Association. The enhanced colour and/or clarity of heat-treated gemstones is permanent. This also may be referred to as "heat only," "low heat" or "gentle heat." ImpregnationThe impregnation of a porous gemstone with a colorless agent (usually plastic) to improve durability and appearance. Turquoise commonly is impregnated. The treatment is considered permanent but does require special care. IrradiationThe use of neutrons, gamma rays or beta particles (high energy electrons) to alter a gemstone’s color. The irradiation may be followed by a heating process. Stability of the treatment varies based on the type of material. Topaz, morganite and spodumene are commonly irradiated to enhance color but may fade if exposed to light or heat. Waxing/OilingThe impregnation of a colorless wax, paraffin or oil in porous opaque or translucent gemstones to improve appearance. Emeralds are always oiled to improve clarity and should not be cleaned with harsh chemicals or ultrasonic cleaners. |
||||||||||||||||||||
Synthetics and Simulants |
||||||||||||||||||||
| Usually the first question a consumer will ask when considering an item for purchase is, "Is it real," when the question should be, "Is it natural or lab created?" By definition, a synthetic gemstone has the same chemical, physical and optical properties as its natural counterpart but is created in a lab. So, for example, synthetic ruby is "real" ruby, but it should never be represented as "natural." Because they are created under controlled conditions, synthetic gems don't have the same inclusions as gems formed by nature in the earth, which is why some people think they are "too perfect." However, because they crystallize in a different way in the lab than in nature, they do have characteristics which a trained gemmologist can generally spot quite quickly under proper magnification. Synthetic gemstones regularly seen in the jewellery market include alexandrite, sapphire, ruby, spinel, emerald, aquamarine, amethyst and opal, among others. A simulant (or imitation) can be any material, natural or synthetic, which is intended to simulate another material. The most familiar diamond simulant, cubic zirconia, has no naturally occurring counterpart. An increasingly popular diamond simulant, synthetic moissanite, does have a natural counterpart, but it is so exceedingly rare that all the moissanite on the market is synthetic. Synthetics and simulants can offer the consumer the look of natural gems at a fraction of the price, and certainly have their place in today's market, as long as they are accurately represented as such. |
||||||||||||||||||||