I have a couple of hobbies that have products that might work for honing, at least along the same lines as cocoa powder. I keep fish and my wife keeps Chinchillas. What these have in common is a fine powder for cleaning. Chinchillas bathe in dust to remove oils from their fur. I just checked this dust and it is exceedingly fine. Here's a link to a picture of the product. Any large pet store should carry this in their small animal section. The other product, which may not be as easy to come by, is diatomaceous eath used in a diatom filter. It is used to remove particulates down to the 3 to 5 micron level. Apparently it can also kill bugs quite effectively by scratching their exoskeletons, thus causing the insect to die of dehydration. Anyway, here's a link to some information about that. Chances are you can find this at any tack and feed shop or a store specializing in fish. Just points to ponder.