I found a rock a little while ago (I can't remember where from, but we stuck it in a garden, where it was quickly covered by foliage, I recently re-discovered a large chunk of it), it appears to be similar to shale or slate, I can see lots of small reflections from the rock in the light, and also from the slurry it generates. I'm guessing they're small quartz crystals. I was worried that the reflections may have been from the way I polished the rock, but the slurry has the same reflections (which is a good sign) and dries to a chalky powder (I don't know if this is good or bad or if it doesn't matter).

I cut a piece of it into a rough whetstone to test it on a pocket knife. About 100 laps applying about 1-2 Newtons of force (hey, it's just a SS pocket knife...) gives a shiny edge (I can't tell if it's a mirror finish or not, but it appears to have removed very little steel).

I think I've had enough success with this rock (it does cut steel, and it's definitely >1k) to justify making a proper sized hone from it.

Ask for more information, I can probably find out about it.

Here are some photos of the rock. Click the photos to open full sized images.