Please help!

[Ajroar]

Hi, my 11 year old son has had 2 pimples come up in the last 3 months, one on his side and one on his toe. As the first one wouldn't go away and only became worse I decided to break the top and release the pus... There was no pus! But with tweezers I was able to remove a solid green, glass like shard from the core. The crater left behind did not bleed and then healed over. The same thing happened with the next one 2 months later. What is it and what's causing it? Please, some ideas would be great