Monday, January 13, 2014

Smithsonian Gemstones II

More than 3,000 years ago Egyptians fashioned beads from golden green crystals mined on an island in the Red Sea. Known to the Greeks and Romans as Topazios, this island off the coast of Egypt was an important sources for fine peridot, the gem variety of the mineral forsterite. Originally called topazion, after the island, this gem was renamed peridot in the 18th century. The island is known today as Zabargad, the Arabic name for peridot.

Other sources of peridot include Burma, the United States, Norway, Brazil, China, Australia, and Pakistan.
All members of the garnet family share the same basic atomic structure and are closely related, but they differ in chemical composition. The color of a garnet is determined by its composition. Garnets exhibit the complete spectrum of colors: spessartine (yellowish to reddish-orange), almandine (red to brownish or purplish-red), pyrope (red), grossular (colorless, pink, orange, yellow, brown or green), and andradite (green to greenish-yellow). Rhodolite garnet is a variety that is intermediate in composition between almandine and pyrope and is typically pink to purplish-red in color. These raspberry pink garnets were found in North Carolina in the late 1800s and were named rhodolite because the color resembled the blossoms of the local rhododendron plant.

The most important sources of top quality rhodolite garnet are Tanzania, India, Zimbabwe and Sri Lanka.
Quartz is one of the most abundant minerals in the Earth’s crust. It is composed of the elements silicon and oxygen and in its pure state is colorless. Small amounts of various impurity atoms can produce a range of colors in quartz.

Amethyst, the most valuable gem variety of quartz, is purple. Just a few atoms of iron replacing some of the silicon will cause the purple color. Most gem amethyst is found in Brazil and Uruguay.
The tourmaline family consists of more than ten distinct minerals, but only one, elbaite, accounts for nearly all of the tourmaline gemstones. Although best known in shades of green and red, elbaite also can be blue, purple, yellow or colorless. In the late 1990’s, copper-containing blue tourmaline was found in Nigeria. The material was generally paler and less saturated than the much prized Brazilian “Paraiba” stones, but the Nigerian gems typically had fewer inclusions.
Pure spinel is colorless, but impurities give rise to a range of colors, most typically pink or red, but also purple, green and blue. Spinel is a magnesium aluminum oxide and forms when impure limestone is altered by heat and pressure.
They are commonly found in occurrence with corundum (ruby and sapphire) and have historically been confused with each other due to their many similarities. Both minerals are hard, yield durable gems, and form in a range of colors. The major sources of spinel gemstones are Burma, Sri Lanka, and Thailand. Other significant occurrences are Pakistan, Afghanistan, Tajikistan, Tanzania, Viet Nam, and Russia.