Review on Premium 50-Piece Titanium Engraving Milling Carbide Set: Superior Precision and Durability by Sean Ceo

The good news is they came and round coated drill bits were the only ones I've had any luck with. The coating matures quickly (even with continuous water). If you are polishing/preparing a gem, rocks or minerals with a hardness above 7 they will not work for you. Get bits that have a diamond inside the bit.