Cooking LR Question

[Phanman]

Ok... so in anticipation of my new 125 gallon tank which will be setup in Febish. I bought 45lbs of LR to cook for a few months in a plastic garbage can. I first bought the rock, and everything looked normal. Placed the rock in a rubber tub at first with salt water, a powerhead, and heater. I let it sit for 2 weeks (man did the water stink like crud), and mixed new salt water in a garbage can with a heater and powerhead. I decided to scrub down the rock before making the transfer to the garbage bin, but noticed that some of the rock had turned black on one side???

Is this normal? some of it (black stuff) seemed to scrub off, but most of it is on the rock still. What is this stuff? Its not slimy or stringy or anything. Comes off like soot...am i destroying my new LR or something?

thx,
John