Gemstones are of tow kinds. There are those that are mineral in origin and are formed as crystals that grew underground during the formation of our earth. They are generally hard and heavy, have beauty and performance and if they are rarely found, they become classed as precious stones. Of these diamonds, emerald, ruby and sapphire are the most popular. Other rare and expensive stones are opal, alexandrite, and newly discovered tanzanite.
An austrian mineralogist named F.Mohs developed a scale to measure the hardness. This so called as Mohs Scale goes from one to ten, the softest being talc with a hardness of one and the diamond, the hardest known stone given a measure of hardness ten. The hardness is measured by scratching the sample with a diamond point and examining the depth of the resulting wound. A second parameter is the heaviness or specific gravity but this is merely the density of the gemstone and not related to the hardness.
Of the organic stones, the rarest are amber and corel but these are not very popular in India and Sri Lanka. There are over thirty popular varieties of precious and semi-precious gemstones in a range of colors, many of which are rare collectorâ€™s items.
The usual practice is to cut the rough stone into a desired shape and polish it to obtain the best luster possible.It is then set into a metal frame that holds the gemstone securly and enhances its qualities. The metal generally used include silver, gold and platinum. Pure gold of 24 karat and sure silver of 100 percent are very soft and gems set in these pure metals can easily fall out when the piece of jewelry is in regular use. Jewelers therefore prefer to mount precious stones in gold of 22 karat or in sterling (92.5 percent) silver.