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
Mineral spa Mineral spa Mineral spas are spa resorts developed around naturally occurring mineral springs. Like seaside resorts, they are mainly used recreationally although they also figured prominently in prescientific medicine. Spas were used for millennia for their purported healing or healthful benefits to those wealthy enough or close enough to partake of their waters. This was called a mineral cure and gave let to such phrases as "taking a cure" and "taking the waters". There has always been a mixture of recreational and medicinal connotations involved, from rest and relaxation, stress relief, and convalescence to more specific notions such as humorism.