What are the black/brown spots on this weld ?

I've been wondering what those are for awhile, are this impurities coming up or something?

It feels like glass and is really sharp, I once got one bit stuck in my finger through my glove lol

I've been wondering what those are for awhile, are this impurities coming up or something?

It feels like glass and is really sharp, I once got one bit stuck in my finger through my glove lol