Diamond
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Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, but diamond is metastable and converts to it at a negligible rate under those conditions. Diamond has the highest hardness and thermal conductivity of any natural material, properties that are used in major industrial applications such as cutting and polishing tools. They are also the reason that diamond anvil cells can subject materials to pressures found deep in the Earth.
A naturally-cut diamond crystal
Main diamond producing countries
One face of an uncut octahedral diamond, showing trigons (of positive and negative relief) formed by natural chemical etching
The extreme hardness of diamond in certain orientations makes it useful in materials science, as in this pyramidal diamond embedded in the working surface of a Vickers hardness tester.
Refractive index
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In optics, the refractive index of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
Diamonds have a very high refractive index of 2.417.
A split-ring resonator array arranged to produce a negative index of refraction for microwaves
In optical mineralogy, thin sections are used to study rocks. The method is based on the distinct refractive indices of different minerals.