Im not sure what they are but I used to get them too. Most notably in my shoulders.. could never see anything, but you can feel them and they feel very similar to a grain of sand. you move your hand over a spot a few times and they come out..
Never knew what they were either.. I always figured it was my body just putting out some type of impurity.. Dont get them as much as I used to when I was a teenager. And of course I tend to eat healthier things nowadays that im 45 lol