Gemstones
It is important to distinguish the notions gem and gemstone. The word gem should be applied to the minerals which are valued for their hardness, rarity and shine. This term is more wide than "gemstone" which is used in more restricted sense and applied to precious stones after they have been polished and cut.
Gemstones as precious minerals form one conventional group whose main representatives are diamonds, emeralds, sapphires, rubies and opals. However there are other groups of gemstones along those that are precious - they are formed by semi-precious gems like topaz, zircon, chrysolite, spinel and others and usually called fancy stones.
The main and essential property of each gemstone is the degree of hardness. Scientific mineralogy resources may explain that varying hardness is registered as a scale with levels differing from 10 to 1 in descending order. The exceptional hardness of a diamond is considered the top one and 10 pts are assigned. Then follow corundum gemstones - ruby and sapphirewith the value of 9 pts. Topaz has degree of 8 and quartz of 7. The degrees of hardness below 6 have those gemstones that can be scratched with a good knife. Besides the hardness gemstones may differ in weight and density, cut and clarity, color and shape.
The color of a gemstone may be discharged by a heat, so be careful and try not to expose your jewelry to fresh air and sunlight. When exposing gemstone to heat you may alter the color of some of them so readily that it could be used in commercial aims. So sherry-yellow topaz gemstone is rendered pink, amethystine or smoky quartz becomes yellow, and colored zircons may be decolorized in order to resemble diamonds.
I want to add that some gemstones appear to change color from the candle light and from almost all other artificial means of illumination. After that some amethysts become inky, some sapphires acquire a murky tint, but others may become amethystine.
There are mineral and organic gemstones distinguished. Mineral gemstones are naturally growing rocks found in the earth's surface. There are about 16 groups of different mineral gemstones: Beryl, Corundum, Chrysoberyl, Feldspar, Diamond, Jade, Garnet, Opal, Lapis Lazuli, Quartz, Peridot, Topaz, Spinel, Turquoise, Tourmaline, and Zircon. Organic gemstones are not as durable as gemstones from minerals, and consist of four groups - amber (fossil resin found in the sap of ancient tree pines), coral (is crafted from a small marine animal known as a coral polyp), jet (black coal) and pearl (is formed inside an oyster, or any other mollusk).
From all these gemstones are formed five groups of precious gemstones. They were once referred to as precious stones primarily because of historical and ceremonial use while other stones were referred to as semiprecious stones. These precious gem stones are diamond, ruby, sapphire, emerald, and amethyst.
Today different online stores and services have even more possibilities than jewelry markets in helping you choose the gemstone suitable for you, no matter isi it precious or semi-precious, of organic or of mineral origin.