Where does one purchase these rejects?

Seconding the request on where to purchase rejects. I've always wanted to make jewelry for mom, who has super particular taste in style, but would genuinely prefer something like rejected lab made

No! good question though, but it misses a detail of their molecular geometry/energy structure.

They're both allotropes (different forms) of carbon but the way they are arranged atomically is different. Like soot is basically just random balls of carbon (??? bonds), graphite is rings bonded in a plane (3 bonds, a single layer of which is called graphene), and diamond is a network that extends in 3 dimensions with each atom bonded to 4 carbon atoms.

When you grind them up you are more making smaller and smaller shards of the base material than changing it's energetic structure, although at the edges it is changing. If you grind pathologically I suppose you would eventually reduce it all to soot.

The physical properties of stuff is in part altered by the surface configuration when we get extremely small, but in general we just consider the bulk as even tensie tiny fragments of stuff are thousands (tens, hundreds of depending on geometry) of atoms, of which only a handful have different bonds at the edge.