Chalcedony is a fairly common stone that has been used for centuries for tools and adornment. It is a form of quartz, called micro-crystaline simply meaning that it is very fine grained. Since it is very hard, it was a popular choice among prehistoric types for making ax heads and weapons. Today it is used primarily as a gemstone.
Although it is a very common stone, you may be unfamiliar with the name chalcedony. By the way, if you've read the word but never heard it pronounced, you might be surprised to learn that it is pronounced cal-SID-nee. The reason you may not have heard much about it before is that different colors of chalcedony are given different,more glamorous sounding names. For instance, I'm sure you've heard of Carnelian. Carnelian is red chalcedony. Onyx is black chalcedony. Green chalcedony is called chrysoprase or chrysophase. Banded chalcedonies are usually called agates. Recently though, some of the other, more exotic colors are getting more attention and used as gemstones. And they are just being called chalcedony. Blue chalcedony has been popular for a while now. And Indonesia yields some gorgeous purple chalcedony that is the color of amethyst as well as some beautiful bright yellow specimens. Nice clean, clear pieces of this purple or yellow material sell for $20 per carat or more.
A lot of chalcedony is dyed or otherwise treated to enhance its color. Most of the carnelian on the market is heat treated or dyed and almost all onyx dyed to make it blacker.. There is also some very pretty, turquoise blue chalcedony being sold that are recieving a diffusion treatment. The important thing is to always ask before you buy. Gemstone treatments are ok as long as they are disclosed. Natural, untreated chalcedony is much more valuable than dyed stones and the color is permanant. The color of treated stones sometimes fades over time. I have seen beautiful blue chalcedony turn grey after being exposed to light for a few months.
Chalcedony is in many ways a perfect gemstone. The colors are bold, striking and wide ranging. Nice specimens have just the right amount of translucency, so they almost look like they are glowing from within. And they are extremely durable! Best of all chalcedony gemstones, for the most part are affordable.