Natural gems found in the earth are rare and the process of extracting gems is also very difficult. This makes them very expensive. Jade Bangle In modern times gemstones are identified by gemologists, who describe gems and their characteristics using technical terminology specific to the field of gemology. Jade Necklaces The first characteristic a gemologist uses to identify a gemstone is its chemical composition. For example, diamonds are made of carbon and rubies of aluminium oxide. Jade Jewelry Next, many gems are crystals which are classified by their crystal system such as cubic or trigonal or monoclinic. Another term used is habit, the form the gem is usually found in. Jade Rings For example diamonds, which have a cubic crystal system, are often found as octahedrons. Gems are characterized in terms of refractive index, dispersion, specific gravity, hardness, cleavage, fracture, and luster.