The following information is from the website: http://en.wikipedia.org/wiki/Chalcedony
Chalcedony (pronounced [kælsɛdəni]) is a cryptocrystalline form of silica, composed of very fine intergrowths of the minerals quartz and moganite.[2] These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, whilst moganite is monoclinic.
Chalcedony has a waxy luster, and may be semitransparent or translucent. It can assume a wide range of colors, but those most commonly seen are white to gray, grayish-blue or a shade of brown ranging from pale to nearly black.
The mineral gets its name from the town of Chalcedon.
The hardness (Mohs scale) : 6 - 7
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Various colors of chalcedony from stores of www.amazon.com