Moissanite and carborundum--what's the difference?

[don_willard]

Is the crystal structure of diamond, ie., the tetrahedron with a atom at each point of the tetrahedron found in silicon carbide and if so, does each tetrahedron have two carbon atoms and two silicon atoms, for instance, or what? And is carborundum, the old man-made abrasive, the same as silicon carbide and are they both the same as moissanite when it comes to their crystal structure? I have a large piece of carborundum from a carborundum factory and it is basically black with iridescent large hexagonal crystals here and there and the rest is I guess the term is cryptocrystalline. If silicon carbide or carborundum is diamond structure with the tetrahedra and all, then why are the crystals of my piece of carborundum flat hexagons? signed Confused. Oh and one more thing, why is moissanite gems so unbelievably expensive when a chunk of carborundum isn’t if they are the same?

[SpoilerVirgin]

Check out The Story of Moissanite.

Gems have been made of carborundum for many years. However, the material produced by Acheson’s process has two severe limitations as a source of gemstones. First, the gem-quality crystals produced are never very large, usually much smaller than a cubic millimeter. Second, over a hundred different crystal forms are produced in the Acheson process. These different forms include three crystal forms (cubic, hexagonal, and rhombohedral), which occur in dozens of different polytypes. Finally, the rare gems cut from the occasional large carborundum crystal are usually strongly colored, most often brown or yellow.

[Napier]

“No result found.”

I think these are all nominally the same compound, and the differences are whether they’re single crystals or polycrystalline, and how they’re arranged. And also whether the origin is synthetic or natural.

I got interested in silicon carbide because it’s available as a gemstone AND it has a higher index of refraction than diamond. In visible wavelengths it’s the highest index of refraction I know. So I got a pea-sized artificial silicon carbide gemstone. I presume it is a single crystal. It’s cut in a classic diamond shape. And the thing is perfectly transparent and beautiful, with obviously a very high index, strong reflections on its surfaces and lots of what I think gem experts call “fire”.

[MrDibble]

A crystalline zombie, but nevertheless:

@don_willard

Is the crystal structure of diamond, ie., the tetrahedron with a atom at each point of the tetrahedron found in silicon carbide

Kinda - a cubic structure can occur, but by far the most common form of SiC is in the hexagonal system, specifically wurtzite structure.

@don_willard

And is carborundum, the old man-made abrasive, the same as silicon carbide and are they both the same as moissanite when it comes to their crystal structure?

Yes.

[Jackmannii]

They’re not the same.

No one ever says “Illegitimi non moissanite”.

[Spectre_of_Pithecanthropus]

[bold]Jackmannii[/bold], bravo!

I only want to add: With regard to moissanite, because of its hexagonal crystal structure, i.e. like any crystal system other than cubic/isometric, there will be some double refraction. This means that the back facets will be doubled and hence a bit blurry. As far as diamond simulants are concerned, part of the reason Cubic Zirconia (CZ) was such a big deal-- and still Is the gold standard for diamond simulants–is exactly the “cubic” part, as the substance crystallizes in the cubic system. This is a significant component of why CZ is so successful as a diamond simulant.

The other point about simulants generally is that, just as rhinestones and glass look fake because their optical properties don’t measure up to those of diamond, a simulant can look equally fake if its optical properties exceed those of the gemstone it’s intended to simulate. For example, a green CZ will have greater luster and dispersion than an emerald, and for exactly that reason it’ll look fake. There’s nothing wrong with that if it rocks your boat, though; go for it.