Description
Jade is the common name for two minerals used as gemstones: jadeite and nephrite. Fake jade also exists in the economy, but is less valuable than genuine jade. The two can be distinguished by the fact that genuine jade changes color over time after years of contact with human skin. Jade comes in many different colors: green, brown, black, or cloud white, ranging from very light to very dark. Like soapstone and chalcedony, jade has been cut and worked by the Chinese for thousands of years. Originally used as tools, jade became popular as jewelry during the Han Dynasty. The only place where nephrite is found in China is in Xi'an, near the city of Khotan. In Chinese culture, only worked jade is considered valuable. A lump of unworked jade is therefore of lesser value. The Sanzijing mentions it as a comparison to education: "If one does not polish jade, it is not a precious thing. If a person does not learn, his life has no meaning, for he does not know the norms, values, and respect of life and will therefore never become talented" (玉不琢不成器人不學不知義). Jade is considered a valuable stone that wards off evil forces. If one wears a piece of jade on one's body for a long time, for example, on a red string around the neck, one will see the jade slowly change color. During the 2008 Beijing Olympics, the medals were adorned with jade.
The mineral marcasite, or spear stone, is a pale bronze-yellow to tin-white iron sulfide with the chemical formula FeS2. The name is derived from the Arabic word Makraschit, the original name for pyrite and similar minerals. Marcasite is rarely used as a gemstone, but was especially popular in France during the Middle Ages, where it was used as a folk ornament. Today, small cut marcasites are set in silver, especially in Thailand. In prehistoric times, it was used as a fire starter in combination with flint. It is found in hydrothermal, postvolcanic, hypergenic, and sedimentary rocks. Marcasite has the same chemical formula as cubic pyrite but a different crystal structure (orthorombic). This is called isomorphism. Marcasite darkens rapidly in air, and specimens of this mineral can even crumble or disintegrate over time. It is of little importance as an iron or sulfur ore mineral. In the gemstone trade, the name marcasite is incorrectly used for cut pyrite. This "marcasite," with a brass-yellow color and a high metallic luster, is primarily set in silver jewelry as a decorative element. The name marcasite is of Arabic or Moorish origin. It was used for all pyrite-like minerals such as marcasite, chalcopyrite, sphalerite, and others. Marcasite is typically formed hydrothermally and is found in sedimentary rocks (such as limestone, shale, or lignite) or in veins (with zinc and lead sulfide). Marcasite forms at low temperatures and under reducing conditions. It is found in chalk deposits, such as those at Cap Blanc-Nez near Calais, France. It is sensitive to heat and sweat; sulfur dioxide is released in fire. The mineral corrodes at the surface.
