dirt or?

Question
A couple of months ago, I noticed one of my guinea pigs had what appeared to be skinny specks of brown/black dirt particles deep in the hair on his back.....lots of them. I shaved his back really short as they  particles seemed impossible to pull out with my fingers, like they were stuck or something. I noticed today he has the same thing. His hair has all grown back now and I don't know whether to shave it again or what. They aren't jumping or moving so I know it's not fleas, not adult ones anyways. Have you ever heard of this or seen it. This pig is housed with one other pig in a large cage.

Answer
Hi Diana.  Some guinea pigs can have sensitive skin which can give them a slight scurf in their hair.  If this is the case, gentle washing with a mild shampoo is necessary to maintain a healthy skin and coat.  On a more medical note, dandruff flakes in the hair can indicate an aggravant such as mites or more likely with a thick scurf, a fungal infection in the hair.  A good fungicidal shampoo is really needed to tackle the job, and you'd generally need to get a strong shampoo such as Seleen from a vet.  If the specks are not due to a fungal infection then i would next suspect lice (guinea pigs can not get fleas), especially as you have described them as skinny brown/black particles.  To treat lice you can use human headlice shampoo safely.  As your pig lives with another it would be best to treat him also, even if symptoms are not visible.
I hope this helps!