Discovered by Obsius in Ethiopia, obsidian is named after him.

 

Obsidian is a natural glass of volcanic origin that is formed by the rapid cooling of viscous lava. Obsidian is extremely rich in silica (about 35 to 80 percent) and is low in water. Obsidian is made of the same minerals as granite but cooled so quickly that they do not have time to crystallize. 
Obsidian has a glassy lustre and is slightly harder than window glass. Though obsidian is typically jet-black in color, the presence of hematite produces red and brown varieties, and the inclusion of tiny gas bubbles may create a golden sheen. Obsidian in attractive and variegated colors is used as a semiprecious stone.

 

Obsidian has been used for thousands of years for toolmaking. In 1967 archaeologists working at the site of Tlapacoya, southeast of Mexico City, uncovered a well-made blade of obsidian associated with a radiocarbon date of about 21,000 BC. 
Sharp shards of obsidian were formed into arrowheads by Indians, who obtained large quantities of obsidian from Obsidian Cliff in Yellowstone National park. The Aztecs used a great deal of obsidian for tools, including sacrificial knives, the eyes of carving of their gods, and even mirrors. 
Obsidian is used for dating. Obsidian 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. 
Obsidian has also been used for jewelry for centuries.