Gemstone IMMs and other free sets from the artist.
Michael Dunnam has tons of useful stuff for game developers on his Gumroad page. For example, look at these free shiny IMM brushes: 17 Accurate gemstone IMMs, all wrapped up in one convenient insert multimesh brush.
Get the brushes
You can also check out his other IMM brushes. Here are some of the sets:
1 of 2
1 of 2