What's the reddish powder that oozes out of a black rock? As an artist, some of the work I do involves the use of seashells, sea rocks/stones, etc. 
I picked up a black rock from shore at one of the beaches here in Florida and it looked somewhat clumpy with other rocks and debris encrusted on it. As I do with everything I collect, I thoroughly washed all the rocks with fresh water and then placed them in a bucket filled with water and a good dose of bleach.

After drying the rocks, I noticed that the black one had small, reddish, powdery pimples all over it. It looked odd, so I broke the rock in half with a hammer,but there was nothing red inside. It actually looked like carbon, but the rock isn't brittle or powdery. I stuck the two halves back in a bucket, filled it half way with water, and them put more bleach in it. The water turned dark, almost muddy, and a reddish, almost gelatinous precipitate appeared and it was quite a bit of it. Any idea what the heck is in that rock that can cause that? I didn't notice any odor other than the bleach, albeit the rock did smell a  bit sulfury. Any idea what I have soaking in the bucket?
Is your rock magnetic?  It might not be, but what you are reporting sounds like iron oxide (i.e. rust), and so your rock could be an iron ore like taconite, or even magnetite.  Such an ore could look like carbon.  Even if not magnetic, it could still be an iron ore.