Black specs in baby bottle?

We use Similac Pro-Sensitive pre-mixed formula for our 10wk old boy. Today we noticed tiny black specs inside the Avent glass bottle. We have a thorough cleaning and sterilizing routine with our bottles and none of the other bottles have those specs. I touched one and it smeared inside the glass almost like a spec of coal. Does this sound familiar to anyone? Unfortunately, I didn’t think to take a picture before we dumped that bottle and sterilized it.