Do you know when a mineral become a gem? A mineral can claim the title of the gem if, in its natural state or when cut in certain ways, its properties are thought to be beautiful. Gems are those minerals which are prized throughout the world for their eye-pleasing appearance.
Some gems are very precious indeed. Diamond, for example, has exactly the same chemical composition as graphite, the soft lead-grey material which is used in pencils. Why, then do we value diamond so highly?
There are two main reasons. Firstly, diamond has a very high refractive index. This means that although diamond is glass clear, it tends to bend the light passing through it much more than glass does. If the diamond is cut in certain ways, it gives a delightful play of color which no other gem can rival.
Indeed, this is the sole reason for cutting a diamond. It is scarcely recognizable in its raw state, and it would certainly be hard to believe that uncut stone and such gems as the famous Kohinoor diamond were the same material. Diamonds are also the hardest known mineral.
This has led to its wide use, not him cutting the hardest stones or metals. Now, however, diamonds suitable for industrial use can be manufactured artificially, but nature is still needed to furnish us with gem quality stones.
Another property which gives a stone its value is the rarity. For example, even if the correct materials are present, it takes conditions of extreme heat, and pressures equivalent to being buried as much as 120 kilometers beneath the surface of the Earth to form a gem.
Other true gems are the deep, green emeralds, the transparent red ruby, and the delicate to deep blue sapphires. Emeralds are a variety of a mineral called beryl which occurs associated with granites.
The ruby is a type of corundum which you may know as the hard stone that is used to sharpen knives and other cutting edges. Sapphire is also a variety of corundum. All these gems are rare, resistant to scratching, beautiful in color, have an attractive luster and can be but to further enhance their beauty.
As well as these very valuable stones there are other less valuable, but in some ways just as attractive, stones that are known as semi-precious. These tend to be less hard or brilliant in color and luster, and rather more common.
Many varieties of quartz are semi-precious, such as citrine, amethyst, and the non-crystalline opal; the banded ones such as onyx and agate are also very pleasing. Even volcanic glass called obsidian, or the type of soft coal usually called jet, can be polished and are often used in jewelry.
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