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3D3DHafnium carbide cutting insert.
This might be a hafnium carbide milling bit insert. Tungsten carbide inserts that look exactly like this are very common, and hafnium carbide is really hard too.

Arguing in favor of its being hafnium carbide is the fact that it came in a box prominently labeled in several unmistakable ways as HfC:

Arguing against is that I can find no record of anyone ever having made such inserts, and the company, Hafnium Carbide Corp, does not appear to exist at the given address, or anywhere else for that matter.

Hafnium coated, as it says on one side of the box, is perhaps more likely, but I prefer to think they are exotic solid hafnium carbide created as an experiment that almost made it to market, but not quite. This is largely wishful thinking on my part.

Source: eBay seller hemlytool
Contributor: Theodore Gray
Acquired: 24 March, 2009
Text Updated: 24 March, 2009
Price: $50
Size: 1"
Purity: 93%