Diamond has an unusual adamantine luster, a brilliant, almost metallic surface appearance that, along with its extreme hardness, distinguishes diamond from all other minerals. Quartz is softer and has a glassy luster. (Learn more about luster) The word "adamantine" refers to another ancient name for diamond, adamant. Today adamant does not signify diamond but extreme, diamondlike hardness.
Diamonds form at great depth, well below the crust. Normal geologic processes bring diamonds to the surface too slowly to avoid retrograde metamorphism, which turns diamonds into graphite. They're able to reach the surface only in peculiar eruptions that explode into the air, leaving behind bottomless pipes of mantle rocks called kimberlites. See this article for more about diamond geology and this one for more about contraband "conflict diamonds."
Diamonds occur in a range of forms, from nearly perfect crystals to rough grains like this one to warty black lumps known as bort. All forms of diamond are valuable, either as gems or as superhard grinding and cutting grit. Gem diamonds are used for their hardness and transparency in the diamond anvil cell, an apparatus that puts mineral samples under high pressures and temperatures.
This diamond, like most diamonds mined today, is from Africa, mined in the Tshikapa gravel beds in Kasai province of the Democratic Republic of Congo (formerly Zaire), near the Angolan border. Other diamond-producing regions include Australia, Canada and Brazil. It was cheap enough that I have actually used it to scratch other minerals.