This wide collection of hard, colored, vitreous to opaque stones (jaspers), is very famous since antiquity. Names like Red jasper, Blood Stone, Chrysoprase, Petrified Wood, Carnelian and Agates are just to list few from this big family of stones used by the Greeks and the Romans in cameos to celebrate their famous faces. Technical/details: SiO2 - Silicon Dioxide Agate, like Chalcedony, is a variety of quartz gemstones composed of layers of quartz, sometimes of different colors. Agate usually occurs as rounded nodules or veins in rock such as volcanic lava. The layers of quartz are often concentric. The composition of agate varies greatly, but silica is always predominant, usually with alumina and oxide of iron. Colors:Agate comes in most colors. There are several types of agate. Common ones are: blue lace agate, moss agate, tree agate and petrified wood. Care and Treatment : Since agate is porous, it is often dyed to enhance its natural color. Often treated agate is sold as Black Onyx. Petrified wood is fossilized wood that has had its organic matter replaced by agate. Hardness: Agate will chip and crack rather easily though it has a hardness rating of 7.