Minerals used in jewelry are called Gemstones. For durability, minerals should score a 5 or better on the mohs hardness scale. Minerals scoring a 4 on the hardness scale can be used for necklaces and pendents but must be worn with care.
GEMS: PRECIOUS AND SEMI-PRECIOUS
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Minerals that are used in jewelry historically have been divided into two categories, precious and semi-precious. Generally, precious gemstones are those gemstones perceived to be rarer and command a higher price in the marketplace. Most people would place corundum (sapphires, ruby), diamond, peridot and beryl (aquamarine, emerald), in the category of precious minerals. In ancient Greece, amethyst was so rare it was considered a valuable precious gemstone. There is no definitive definition separating precious stones from semi-precious stones.
Some additional minerals used as gemstones:
Many collectors of gemstone minerals have no intention of making jewelry out of them. They seek the natural form and beauty of the mineral in its raw, unmodified, state.
Amethyst, a variety of quartz
The purple color of natural amethyst is cause by “natural” irradiation of iron impurities in the quartz. Clear quartz can be made purple by irradiating it after artificially introducing iron impurities.
Many beautiful minerals are too soft to be used in jewelry
Rhodochrosite on quartz
Rhodochrosite has a hardness of 3.5 to 4, it is very difficult to facet and rarely used in jewelry. Quality crystals of Rhodochrosite on matrix is rather expensive.
The apatite crystals above have a hardness rating of 5, which makes it one of the softest gemstones used in jewelry.
Scoring a 4 on the hardness scale fluorite will not hold up long as a gem for a ring. Sometimes fluorite will be faceted/polished, to show off their beauty, and the skill of the lapidaries. Faceting and polishing a fluorite is very difficult due to its softness and its cleavage planes. When backed by a base metal, pendents made from Fluorite can be visually striking.
The bright red realgar crystal (often called “ruby sulfur” looks like it would be a great gem stone but it has a number of features undesirable for a jewelry piece. First, it has a hardness of 2-1.5, secondly, it is chemically arsenic sulfide, and lastly exposure to light for an extend time results in the crystal literally falling apart to form a yellowish powder.