Well the SG on that unknown stone is ~3.23, which means it's incredibly heavy for how large it is (and I can attest it feels that way). I also measured the washita and it's ~2.31, which I'm very excited about.
Ok so... 3.23 is wildly higher than any stone I've tested, with the exception of Coticules which can get there because of the garnet. Given that yours clearly isn't a coti, then looking here:
Density, or Specific Gravity Chart, is a property that is bulk of material. Specific Gravity is the ratio expressing the extra weight of a mineral when
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There's not much else (afaik) that gets made into whetstones which it could be, apart from:
Jadeite @ 3.3. And it's probably fair to say it's not that either.
Corundum @ 3.9. This is the natural form of Aluminium Oxide that 'Alundum' synthesised, so it could be an AlOx based synth.
or... the SG of Silicon Carbide is 3.22. It could be a very compact / pure SiC based synth.
Most likely a combination of the two I guess - some kind of vitrified SiC+AlOx stone.
Pretty unusual whatever way you spin it! The SG of Norton Crystolon or India stones is far lower.