The dark spots are some sort of mineral coating, lots of shiny fine particle powders, blacks, Browns, reds, golds, oranges, purples, been scrubbed with wire brush and particles are more prominent
Don't start with an assay. Start by crushing a nice vuggy piece into dust. Then pan that to see if there is any VISIBLE gold. If you see gold in the pan, DEFINITELY have it assayed. If no visible gold in the pan, then it is up to you if you want to try an assay.