Obsidian

a volcanic rock which is formed by the rapid cooling of a molten rock with a composition of granite. It is generally glassy, black with a vitreous (glassy) lustre and in composition contains a lot of silica (SiO2).

Taken from Dictionary of Science

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Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock. It is produced when felsic lava extruded from a volcano cools without 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. Because of the lack of crystal structure, obsidian blade edges can reach almost molecular thinness, leading to its ancient use as projectile points, and its modern use as surgical scalpel blades.

Taken from Wikipedia

Mach number

the speed of a body expressed as a ratio with the SPEED OF SOUND. Mach I is sonic speed, and thus below I is subsonic and above I is supersonic.

Taken from Dictionary of Science

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Mach number (Ma or M) (generally pronounced /ˈmɑːk/, sometimes /ˈmɑːx/ or /ˈmæk/) is the speed of an object moving through air, or any fluid substance, divided by the speed of sound as it is in that substance. It is commonly used to represent an object's (such as an aircraft or missile) speed, when it is travelling at (or at multiples of) the speed of sound.

where

is the Mach number

is the speed of the source (the object relative to the medium) and

is the speed of sound in the medium