Obsidian (ub-SIH-dee-in) is a natural glass that forms from volcanic activity. The gemstone is made of the same minerals as granite, but cools so quickly when exposed to air that the minerals do not have time to crystallize. Some inclusions could be bubbles or crystals, creating a random pattern such as snowflake. Obsidian is extremely rich in silica (about 35 to 80%) and is low in water. It may first have been discovered in Ethiopia by a man named Obsius, from whom its name is derived. Obsidian is fairly fragile, slightly harder than window glass. It scratches very easily and sharp blows are likely to crack it. Obsidian has been used for tool-making since at least 21,000 B.C. and in jewelry for many centuries. American Indians used it for arrowheads, and the Aztecs used a great deal of obsidian for items including sacrificial knives and mirrors. Today, obsidian is used for dating; the stone weathers slowly at a uniform rate, and the thickness of the weathered layer is measured microscopically and gauged against known standards to give a date in years. Major sources include Iceland, Italy and the U.S.A. (especially Wyoming), plus Hawaii, Japan and Java.