Chalcedony – (kal-sed-ne)
Chalcedony is similar to agate and jasper as a cryptocrystalline form of silica, composed of very fine quartz.
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.
Chalcedony can occur in many varieties. Many semi-precious gemstones are forms of chalcedony. Some of the more notable varieties of chalcedony are:
Agate
Carnelian
Chrysoprase
Moss Agate
For more information about Chalcedony try these links:
http://stoneplus.cst.cmich.edu/chalcedony.htm
http://www.mindat.org/min-960.html
http://minerals.usgs.gov/minerals/pubs/commodity/gemstones/sp14-95/chalcedony.html