I'm going with Bill on this one. I turn a lot of wormy mesquite, and I
fill most of it with epoxy tinted with copier or laser cartridge toner.
On small holes and termite runs it can look really nice.
However, it can look like black plastic if you have a lot of it,
depending on how far you take your sanding/polishing routine. I am now
adding a little ground coffee to the epoxy/toner mix and it looks more
natural to see the brown flecks in the black. I have also used key
filings from the local hardware store that I crush with a hammer mixed
in with the toner and it looks pretty good if you don't overdo it.
Just about anything will work as long as it isn't too absorbant since
you aren't using the epoxy as an adhesive, just a resin mixture.
I would watch out putting anything in the epoxy and leaving it clear.
After a period of time, in my experience it yellows.