Aggregated diamond nanorod
Aggregated diamond nanorods, or ADNRs, are a nanocrystalline form of diamond, also known as nanodiamond or hyperdiamond. == Discovery == Nanodiamond or hyperdiamond was produced by compression of graphite in 2003 by a group of researchers in Japan and in the same work, published in Nature, it was shown to be much harder than bulk diamond.
Source: Wikipedia — Aggregated diamond nanorod (CC BY-SA 4.0)