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Gemstones

Gemstones are minerals, rocks, or petrified materials that when cut or faceted and polished are used in jewelry. In some cases, they are even organically-based (such as amber, which is fossilized tree resin; and jet, a form of coal).

Gemstones & Info for Buying
 

Gems get their colour from the reflection of light. For example, a ruby collects all the colours of the white light (red, blue, green, etc) and reflects red back to the sun.

Colour is the most obvious and attractive feature of gemstones. The colour of any material is due to the nature of light itself. Sunlight, often called white light, is actually a mixture of different colours of light. When light passes through a material, some of the light may be absorbed, while the rest passes through. The part that isn't absorbed reaches our eyes as white light minus the absorbed colours. A ruby appears red because it absorbs all the other colours of white light - blue, yellow, green, etc. - and reflects the red light to the viewer. A colourless stone absorbs none of the light, and so it allows the white light to emerge unchanged.

Cutting and polishing
The two main classifications are stones cut as smooth, dome shaped stones called cabochons, and stones which are cut by polishing small flat windows called facets at regular intervals at planned angles.

Stones which are opaque such as opal, turquoise, variscite, etc. are commonly cut as cabochons. These gems are designed to show the stone's colour or surface properties as in opal and star sapphires. Grinding wheels and polishing agents are used to grind, shape and polish the smooth dome shape of the stones.

Gems which are transparent are normally faceted, a method which shows the optical properties of the stone’s interior to its best advantage by maximizing reflected light which is perceived by the viewer as sparkle. The facets must be cut at the proper angles, which varies depending on the optical properties of the gem. If the angles are too steep or too shallow, the light will pass through and not be reflected back toward the viewer. Special equipment is used to hold the stone onto a flat lap for cutting and polishing the flat facets.  In some rare cases, some cutters use special curved laps to cut and polish curved facets.

Treatments applied to gemstones

Gemstones are often treated to enhance the colour or clarity of the stone. Depending on the type and extent of treatment, they can affect the value of the stone. Some treatments are used widely and accepted in practice because the resulting gem is stable, while others are not accepted most commonly because the gem colour is unstable and may revert to the original tone.

Heat
Many gemstones are treated by heat, either to improve colour or clairity. Most Citrine is made by heating Amethyst and melting amethyst and citrine together makes amethrine. Much Aquamarine is heat treated to remove yellow tones and give a purer blue. Nearly all Tanzanite is heated to low temperatures to remove brown undertones and give a more desirable blue/purple colour. A considerable portion of all sapphire and ruby is treated with high heat to improve both colour and clarity.

Radiation
Most blue topaz, both the lighter and the darker blue shades such as "London" blue, has been irradiated to change the colour from white to blue. Some improperly handled gems which do not pass through normal legal channels may have a slight residual radiation, though strong requirements on imported stones are in place to insure public safety. Most greened quartz (Oro Verde) is also irradiated to achieve the yellow-green colour.

Waxing/oiling
Emeralds contain natural fissures that are sometimes filled with wax or oil to disguise them. This wax or oil is also coloured to make the emerald appear of better colour as well as clarity. Turquoise is also commonly treated.

Factors influencing esteem
Factors influencing the esteem in which gems are held are attractiveness, durability, rarity, fashion, light reflection and size, but historical rarity can also play a role, as can the matter of whether there are other kinds of gems offering the same or nearly the same colours. For example, all the quartz gemstones--amethyst, citrines, rock crystal, and so on--are quite inexpensive, but amethyst still enjoys higher esteem than its quartz siblings, both because it was once considered rare and precious, and because it remains the quintessential purple gemstone, while the other varieties of quartz are imitative of more lustrous, harder, and rarer minerals.

Synthetic and artificial gemstones

Some gemstones are manufactured to imitate other gemstones. For example, cubic zirconia is a synthetic diamond simulant composed of zirconium oxide. The imitations copy the look and colour of the real stone but possess neither their chemical nor physical characteristics. However, true synthetic gemstones are not necessarily imitation. For example, diamonds, ruby, sapphires and emeralds have been manufactured in labs, which possess very nearly identical chemical and physical characteristics to the naturally occurring variety. Synthetic corundums, including ruby and sapphire, are very common and they cost only a fraction of the natural stones. Smaller synthetic diamonds have been manufactured in large quantities as industrial abrasives for many years. Only recently, larger synthetic diamonds of gemstone quality, especially of the coloured variety, have been manufactured.

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Source: Wikipedia