Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.

It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth. Obsidian is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where the chemical composition (high silica content) induces a high viscosity and polymerization degree of the lava. The inhibition of atomic diffusion through this highly viscous and polymerized lava explains the lack of crystal growth. Obsidian is hard and brittle; it therefore fractures with very sharp edges, which had been used in the past in cutting and piercing tools, and are still used as surgical scalpel blades.

Pliny's Natural History features volcanic glass called "Obsidianus", so named from its resemblance to a stone found in Ethiopia by one Obsius.

Obsidian is mineral-like, but not a true mineral because as a glass it is not crystalline; in addition, its composition is too complex to comprise a single mineral. It is sometimes classified as a mineraloid. Though obsidian is usually dark in color similar to mafic rocks such as basalt, obsidian's composition is extremely felsic. Obsidian consists mainly of SiO₂ (silicon dioxide), usually 70% or more. Crystalline rocks with obsidian's composition include granite and rhyolite. Because obsidian is metastable at the Earth's surface (over time the glass becomes fine-grained mineral crystals), no obsidian has been found that is older than Cretaceous age. This breakdown of obsidian is accelerated by the presence of water. Obsidian has low water content when fresh, typically less than 1% water by weight, but becomes progressively hydrated when exposed to groundwater, forming perlite. Tektites were once thought by many to be obsidian produced by lunar volcanic eruptions, though few scientists now adhere to this hypothesis.

George Makris (August 29, 1920 – October 16, 2005) was the 18th head college football coach for the Temple University Owls located in Philadelphia, Pennsylvania. He came to Temple after coaching the Bolling AFB team where he was highly successful. He took over from Pete Stevens who was winless in his last season. He held that position for ten seasons, from 1960 until 1969. His overall coaching record at Temple was 45 wins, 44 losses, and 4 ties. This ranks him third at Temple in terms of total wins and eighth at Temple in terms of winning percentage.

Makris coached Bill Cosby during his tenure at Temple, a fact that is alluded to in Cosby's skit "Hofstra" on the comedy album Why is There Air?

He played at the University of Wisconsin-Madison as a guard. He died in 2005.