When diamonds are not located within a "kimberlite pipe," and excavated via a hard-rock or open pit mine, they are found in alluvial stream-beds.

Diamond-bearing kimberlite is an ultrapotassic, ultramafic, igneous rock composed of garnet, olivine, phlogopite, and pyroxene, with a variety of trace minerals. Kimberlite occurs in the Earth's crust in vertical, upwardly-thrusting structures known as kimberlite pipes.

The unique chemical and molecular structure of crystalline diamond is what gives this gemstone its hardness, and differentiates it from simple graphite. The name "diamond," which is also known as "adamant," is derived from the Greek adamas, or "invincible," "untamable," and "unconquerable".

A Type 2-A diamond has a hardness value of 167 GPa (±6) when scratched with an ultrahard fullerite tip, and a hardness value of 231 GPa (±5) when scratched with a diamond tip. The material "boron nitride," when found in a crystalline form that is structurally similar to diamond, is nearly as hard as diamond. Additionally, a currently hypothetical material, beta carbon nitride, may also be as hard or harder than diamond.

A diamond's incredible hardness was the subject of curiosity dating back to the Roman empire, where it was shown to combust in scientific experiments, although the reason for its combustion was not understood at the time.

Experimentation during the late 18th century demonstrated that diamonds were made of carbon, by igniting a diamond in an oxygen atmosphere, with the end byproduct of the combustion being carbonic-acid gas, or carbon dioxide.

Within the fields of metallurgy and materials science, the term "toughness" describes the resistance of a given material to fracture when it is stressed or impacted.

Although diamond is the "hardest," and therefore, most scratch resistant mineral on earth, with a Mohs scale rating of 10, its "toughness" rating is moderate, due to its ability to fracture along cleavage planes. By comparrison, sapphire has a hardness rating of 9, meaning that a diamond is 4 times "harder" than sapphire, yet sapphire has a toughness rating of excellent. Hematite has a hardness of only 5.5 to 6.5, but its toughness rating is also excellent.

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