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Special gemological techniques have been developed to distinguish natural diamonds, synthetic diamonds, and diamond simulants.
The word is from the ancient Greek ἀδάμας – adámas "unbreakable".
Some diamonds, known as harzburgitic, are formed from inorganic carbon originally found deep in the Earth's mantle.
In contrast, eclogitic diamonds contain organic carbon from organic detritus that has been pushed down from the surface of the Earth's crust through subduction (see plate tectonics) before transforming into diamond.
Diamond is renowned as a material with superlative physical qualities, most of which originate from the strong covalent bonding between its atoms.
In particular, diamond has the highest hardness and thermal conductivity of any bulk material.
This is because cratons are very thick, and their lithospheric mantle extends to great enough depth that diamonds are stable.
Diamond also has relatively high optical dispersion (ability to disperse light of different colors).
Long residence in the cratonic lithosphere allows diamond crystals to grow larger.
Through studies of carbon isotope ratios (similar to the methodology used in carbon dating, except with the stable isotopes C-12 and C-13), it has been shown that the carbon found in diamonds comes from both inorganic and organic sources.
These conditions are met in two places on Earth; in the lithospheric mantle below relatively stable continental plates, and at the site of a meteorite strike.
The conditions for diamond formation to happen in the lithospheric mantle occur at considerable depth corresponding to the requirements of temperature and pressure.
In the 20th century, experts in gemology developed methods of grading diamonds and other gemstones based on the characteristics most important to their value as a gem.