Pyrite is a natural material that is a dark color with some shades of a golden hue and has fine to coarse granules. This material is not as consistently fine as most of the crushed fillers. There are some coarser granules mixed in. (The "B" in Bear Tooth Woods in the photo is 3mm high.) Sold in a 10 gram bottle that should finish many pens depending, of course, on how large of a void is filled.
I like to use a small screw driver to transfer the material to the void, fill the void with the material then use thin CA glue to secure. You can use a piece of paper under your pen to catch excess material that may slide off the pen. Before turning, make sure the CA glue has FULLY cured.