sand/bone like formations inside skin

[fireflybts]

Last week a noticed a little bump on the outside on my son's arm (he is 7). It looked like a splinter got in, so I removed the top (it looked more like a black head) and attempted to remove the "splinter", but he was in way too much pain. I took him to his doctor, they made a tiny incision with a needle and took a few pieces of what looked like a splinter, but there was too much blood and he was in too much pain, so we were referred to a dermatologist, as there was clearly a foreign body still inside. I expected that they will inject anesthesia and make an incision, but instead we were sent of with antibiotic cream and a follow up appointment in a month. My mom instinct told me there is something more that has to be done. After 2 more days the wound from the picking was gone and it had formed a crust. Today I took the crust off and pressed down with both thumbs on the sides of the bump. after just one attempt, quite a large amount came out - and it was the strangest thing I have ever seen coming out of skin. It looks a lot like sand, or bone, extremely hard. There are several separate pieces, but in quite significant amount. What could that be? it definitely does not seem like sand or rock could have gone inside his arm (I would have noticed a wound that big), so this structure formed inside his skin. I looked under a magnification glass and it looks like sand.