I passed a 3mm piece of something today. it is rubbery, brown, has a black grainy core and is shaped like a shard of glass. can stones be soft?

Black Stone?: Pigment stones - which are typically brown-black- can be soft. The more cogent question is why are you forming pigment stones to begin with? Typically, at your age, a urinary infection, or less commonly, a blood hemolytic process, is to blame. I would follow up with a urologist or your primary care provider.

Kidney stone: If you mean that you passed something in your urine, then it certainly can be a stone. If you still have the particle then bring it to your doctor and it can be sent to the lab for analysis.