Monday, 19 March 2012

Chalcedony--a stone of many kinds

Chalcedony is a micro-crystal (or cryptocrystalline) form of silica, and made of minute ingrowths of quartz and moganite. Chalcedony has a waxy luster, and can be semitransparent or translucent. It has a wide range of colours, and some of the most commonly seen are as follows—white to grey, greyish-blue or a shade of brown ranging from very pale to almost black.

Chalcedonies occur in a wide range of varieties. Some of the more common ones include—
  1. Agates have multi-coloured curves or angular banding. There are many kinds of agate, all different colours, shapes, and sizes. 
  2. Adventurine is a form of chalcedony, you can easily decide that because of it's translucency and the presence of mineral inclusions. It usually comes in silvery green, blue, orange, or brown. 
  3. Carnelian is a clear-to-translucent reddish-brown kind of chalcedony. It's tone can very from very pale orange to an almost-black colour. (Sard is a similar stone to carnelian, but it is brown instead of red). 
  4. Chrysoprase (also spelled chrysophrase) is a green variety of chalcedony. It is coloured by nickel oxide. 
  5. Heliotrope (also called “bloodstone”) is a green variety of chalcedony, and has red inclusions of iron oxide. The inclusions resemble blood, giving the stone this name. In a similar variety, the spots are yellow instead of red, and is known as “plasma”. 
  6. Moss agate has green finger-like inclusions, making it look like moss or blue cheese. It is not a true agate, because it does not have bands. 
  7. Mtorolite is a green variety of chalcedony, and is coloured by chromium (Cr). It is also known as “chrome chalcedony”, and is mostly found in Zimbabwe. 
  8. Onyx is a form of agate with black and white bands. Similarly, agates with brown, orange, red, and white banding is called sardonyx. 
Chalcedony was used in ancient times for different things. In the Palace of Knossos (in Minoan Crete), chalcedony seals were found. People living along Central Asian trade routes used various forms of chalcedony to make jewellery, ring bezels, and beads strikingly similar to Greek and Roman ones. The High Priest had at least 3 varieties of chalcedony on his breastplate—jasper, chrysoprase, and sardonyx.

Chalcedony is found all over the world. In every one of the 50 US states, one form or another has been found. Some states produce better forms of chalcedony than others, but all have uniqueness. For example, two of the places where chalcedony is found is the Great Lakes area—Lake Superior agate, and flint from Flint Ridge, Ohio. Other places include England, Mexico, Brazil, Poland, Madgascagar, China, Australia, Iceland, Russia, Turkey, Czech Republic, and India.
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