7. Make a list of basic properties of rock-forming minerals and speak on each of them.
8. Find 7 basic properties of rock-forming minerals.
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THE
GEMSTONES
As colorful as the rainbow and as sparkling as fine leaded crystal, gemstones have captured the imaginations and desires of men and perhaps especially, women, for ages. The pursuit of gems have become the subject of legends, fairy tales, epics, and major motion pictures ("Romancing the Stone", for one). Today, finer gemstone specimens are available to the average person than at any time in history.
What makes a gemstone?
Generally speaking, a gemstone is a stone that is beautiful, rare, and durable (resistant to abrasion, fracturing and chemical reactions). Some minerals can be very beautiful, but they may be too soft and will scratch easily (such as the mineral fluorite). Fluorite is extremely colorful and pretty but has a hardness of only 4 on the Moh's hardness scale and has four perfect cleavage directions, which make it only an oddity as a cut gem. Others are too common and are given a semi-precious status (such as agate). Most gemstones have good hardness (above 5) and a high index of refraction (the higher the index of refraction the greater the sparkle). All gemstones have some characteristics falling short of perfection though; even the seemingly perfect Diamond has four directions of cleavage.
Most gems are silicates which can be very stable, hard minerals. A few gems are oxides and only one gem, diamond, is composed of a single element, carbon. There are also a few gemstones that are not true minerals (called mineraloids) but are included here: opal, amber, and moldavite. While almost any mineral can be cut in the manner of a gemstone, below is a list of some of the gem kingdom's more prized and recognized members:
Diamond and Zircon. A diamond is composed of nothing but carbon. Though its chemical element is carbon, like graphite, it has a regular and isometric crystal form and is usually colourless and transparent, unlike other minerals composed of carbon. It is the hardest known natural substance. Zircon, which is as bright as diamond, is among the brightest of gems because of its high reflectivity. It is a chemical compound of metal zirconium. The pale-blue zircon is the most popular.
Rubies, Sapphires and Garnet. Mineralogically rubies and sapphires are the same mineral corundum, which is the hardest mineral next to diamond. When this corundum includes chromium, which gives it a redness, it becomes ruby; when it contains titanium and iron instead, and so is blue, it becomes sapphire. Among red precious stones, garnet, unlike ruby, is dark red.
Emeralds, Beryl and Aquamarine. Mineralogically emerald is beryl, which contains a metal called beryllium. The green colour of beryl is due to an extremely small amount of chromium; green transparent beryl is called emerald; beryl which is transparent, pale water-blue is called aquamarine.