When I first saw these pictures the I couldn't help noticing the strange colors. I suspect that's chromatic aberration from the poor optics, but I don't think we can derive any meaningful information about the colors of these stones.
So, I took pictures of some of my hones with what I believe to be much better optics (vintage leica and modern canon) and electronics (canon, made to produce photographic quality). I put them in a new thread.

As far as garnets go - I'm yet to find any on the surface or in the slurry of a thuringian stone. I don't think the cutting media of those is garnets.