Known to the ancient Greeks as ἀδάμας – "adámas" ("proper", "unalterable", "unbreakable") and sometimes called adamant, diamond is the hardest known naturally occurring material, and serves as the definition of 10 on the Mohs scale of mineral hardness. Diamond is extremely strong owing to its crystal structure, known as diamond cubic, in which each carbon atom has four neighbors covalently bonded to it. Bulk cubic boron nitride (c-BN) is nearly as hard as diamond. Diamond reacts with some materials, such as steel, and c-BN wears less when cutting or abrading them. (Its zincblende structure is like the diamond cubic structure, but with alternating types of atoms.) A currently hypothetical material, beta carbon nitride (β-C3N4), may also be as hard or harder in one form. It has been shown that some diamond aggregates having nanometer grain size are harder and tougher than conventional large diamond crystals, thus they perform better as abrasive material. Owing to the use of those new ultra-hard materials for diamond testing, more accurate values are now known for diamond hardness. A surface perpendicular to the crystallographic direction (that is the longest diagonal of a cube) of a pure (i.e., type IIa) diamond has a hardness value of 167 GPa when scratched with a nanodiamond tip, while the nanodiamond sample itself has a value of 310 GPa when tested with another nanodiamond tip. Because the test only works properly with a tip made of harder material than the sample being tested, the true value for nanodiamond is likely somewhat lower than 310 GPa.