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Mineralogical simulation predicts lonsdaleite to be 58% harder than diamond on the <100> face, and to resist indentation pressures of 152 GPa, whereas diamond would break at 97 GPa. [11] This is yet exceeded by IIa diamond 's <111> tip hardness of 162 GPa.
From theoretical considerations, lonsdaleite is expected to be harder than diamond, but the size and quality of the available stones are insufficient to test this hypothesis. [9] In terms of crystal habit , diamonds occur most often as euhedral (well-formed) or rounded octahedra and twinned , flattened octahedra with a triangular outline.
The hardness of synthetic diamond (70–150 GPa) is very dependent on the relative purity of the crystal itself. The more perfect the crystal structure, the harder the diamond becomes. It has been reported that HPHT single crystals and nanocrystalline diamond aggregates (aggregated diamond nanorods) can be harder than natural diamond. [25]
Pure, laboratory-created lonsdaleite is up to 58% harder than ordinary diamonds. [11] [9] These types of diamonds are known as "impact diamonds" because they are thought to be produced when a meteorite strikes a graphite deposit at high velocity. [10] They may have industrial uses but are unsuitable as gems. [12]
Thus diamonds do not exist forever. The conversion from diamond to graphite, however, has a very high activation energy and is therefore extremely slow. Despite the hardness of diamonds, the chemical bonds that hold the carbon atoms in diamonds together are actually weaker than those that hold together graphite.
The collision of a dwarf planet and an asteroid 4.5 billion years ago resulted in space diamonds in meteorites eventually landing on Earth.
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There are two types of reports on materials "harder than diamond". One is experimental work on ADNR, which is diamond, but in a nanocrystalline form. Another is theoretical, on lonsdaleite - needs to be confirmed by experiment, which showed that lonsdaleite produced so far is too defective and has a hardness significantly lower than diamond.