|Hematite and Pyrite
Gemologists use a set of technical terms to refer to luster or the light reflected from a gemstone's surface. These terms include adamantine (like diamond), vitreous (like glass), silky, resinous, greasy and metallic.
The most common luster, typical of transparent faceted gems like sapphire, spinel, beryl, quartz and tourmaline, is vitreous. The strongest or most reflective luster is known as metallic, indicating a reflective surface similar to polished metal. Gemstones with a metallic luster typically have a refractive index of about 2.6 to more than 3.0, substantially higher even then diamond (at 2.4).
Very few gems have a metallic luster. The best known are hematite and pyrite, two iron-based minerals that have often been used for ornamental purposes.
Hematite, an iron oxide, derives its name from the Greek word for blood, since it is red when sliced thinly or powdered. Hematite crystallizes in the rhombohedral crystal system, and has the same crystal structure as sapphire and ruby. Hematite is always opaque and is typically a blackish grey. You will find it cut as cabochons or faceted, and engravings are particularly popular. Hematite was a traditional material for signet rings. When highly polished it can sometimes look like silver.
Hematite deposits are found in the UK (Cumberland), Bangladesh, Brazil, China, New Zealand, Czech Republic and USA (Minnesota). Hematite has also been found on the planet Mars by NASA spacecraft. Since hematite is usually found in aqueous environments on Earth, it has led to speculation that there was once water on Mars.
Pyrite (also known as Iron Pyrite) is an iron sulphide which is usually brass-yellow or gray-yellow, earning it the nickname of "fool's gold." Its name comes from the Greek for fire, since it produces sparks when struck by steel or flint. Historically pyrite was widely used in jewelry, but is not widely seen today. You will find pyrite in carvings, as side stones in ring settings, and sometimes in pendants in a disk-like form with a distinctive radial pattern. Pyrite has reasonably good hardness (6 to 6.5 on the Mohs scale) but is quite brittle.
Pyrite deposit are found in Peru, Bolivia, Mexico, Sweden, Romania and the USA (Colorado).
- First Published: January-06-2009
- Last Updated: March-04-2011
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