Green Turquoise is a natural material that is a light green color and has fine granules. (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.