Synthetic diamond "nanocrystalline" and "microcrystalline" diamond, respectively. Synthetic diamond is the hardest known material, where hardness is defined as resistance to indentation. The hardness of synthetic diamond depends on its purity, crystalline perfection and orientation: hardness is higher for flawless, pure crystals oriented to the <nowiki>[</nowiki>111<nowiki>]</nowiki> direction (along the longest diagonal of the cubic diamond lattice). Nanocrystalline diamond produced through CVD diamond growth can have a hardness ranging from 30% to 75% of that of single crystal diamond, and the hardness can be controlled for specific applications. Some synthetic single-crystal diamonds and HPHT nanocrystalline diamonds (see hyperdiamond) are harder than any known
Synthetic diamond Diamond can be one single, continuous crystal or it can be made up of many smaller crystals (polycrystal). Large, clear and transparent single-crystal diamonds are typically used in gemstones. Polycrystalline diamond (PCD) consists of numerous small grains, which are easily seen by the naked eye through strong light absorption and scattering; it is unsuitable for gems and is used for industrial applications such as mining and cutting tools. Polycrystalline diamond is often described by the average size (or "grain size") of the crystals that make it up. Grain sizes range from nanometers to hundreds of micrometers, usually referred to as