Minerals form under various conditions beneath the earth. Most gemstones form in the top layer of the Earth called “crust”, with a depth of 3 to 25 miles. The mantle consists mostly of molten rock called magma with a solid upper layer. The crust is made up of three kinds of rocks, known as igneous, metamorphic and sedimentary rocks. Gemstone groups and/or species formed from igneous rocks are chrysoberyl, quartz (i.e. amethyst, citrine and ametrine), beryl (emerald and aquamarine), garnet, moonstone, apatite, diamond, spinel, tanzanite, tourmaline, topaz and zircon.
A mineral is defined as a naturally occurring substance that is formed through geological processes and has a characteristic chemical and physical composition, particularly a highly ordered atomic structure that is normally crystalline.
A rock, by comparison, is an aggregate of minerals and need not have a specific chemical composition. In general, rocks are of three types; igneous, sedimentary and metamorphic. Rocks do not have a specific physical composition like minerals.
Most gems are “minerals that have been chosen for their beauty and durability, then cut and polished for use as human adornment.” This definition covers the vast majority of the stones we regard as gems. Gemstones have their own science called gemmology, which is a branch of mineralogy. The concept of a gemstone is based on human value judgements. A gemstone is usually defined as a highly attractive and valuable piece of cut and polished mineral used in jewellery or other decorations.
Beauty isn’t necessarily in the eyes of the beholder. There are gems some don’t consider beautiful, like a gem that may be pink or olive green.
Durability is usually a high priority while buying or choosing a gemstone/s. Pearls are generally considered to have a life span of about a century, that is because they are soft and the simple act of wiping the dust off of them slowly wears the surface coating away.
Opals have been one of the most prized gems throughout history, but they are notoriously delicate. They have water content and, as they dry out, they may crack. Opals are also fragile and can break with the slightest bump.
Cutting and Polishing
For both cabs and faceted gems, the top is called the crown. The widest part of the gem, the part that defines its outline, is called the girdle. Viewed from the side, the girdle is usually fairly thin. The bottom of a faceted gem is called the pavilion. The largest facets are called mains. There are both pavilion mains and crown mains. The large, horizontal facet on the top, the one that acts as a window into the interior of the gem, is called the table facet. Adjoining the girdle are the break facets. Their purpose is to scatter light, creating more scintillation. There are both crown and pavilion break facets. The top rows of facets, those next to the table, are called star facets. Along with the other crown facets, they serve to control the entry and exit of light from the gem. The pavilion facets are designed to reflect the light back to the viewer.
There are four basic styles of gem cutting: tumbling, cabochon, faceting and carving.
A gem material which is completely the product of nature and which is unaltered by humans except for ordinary cutting and polishing.
Any gem material which has been treated to either improve clarity or to enhance its colour by using various treatments beyond ordinary cutting and polishing.
Synthetic gems possess same chemical compositions, structure, properties and appearance as their natural counterpart or seed crystal. They are not made by nature but created by humans.
Similar as synthetic gem but has no natural counterpart or seed crystal.
Any gem material natural or otherwise which is created by assembling two or more pieces of similar or different materials along different junctions.
Any gem material natural or otherwise which has the same look as the gem it imitates.
Most gemstones are treated using various methods either to improve colour and clarity. Some of these methods are generally accepted in the trade today like oiling of emeralds, while other methods are not. The accepted trade methods are the ones that change the colour of the gemstone permanently.
Different Treatments are used for different stones considering their nature and reaction to it. Sapphires are commonly found heat treated as they react well to the treatment on different heating levels so one can get desired result from it. Some of them are
The physical properties, chemical composition and atomic structure of the synthetic gemstone are basically same as those of the natural stones, which they are trying to duplicate.
Various methods or techniques are used to produce synthetic gemstones.