A Blackish hard pebble looking thing was in my arm?

the other day my arm was itchy when i looked at it i seen a black spot on my arm it was kinda hard. the skin was not broken so was picking at it and i popped it when it came out and it looked like a shell of a bug so i tried crushing it snd it was hard like a rock.. when it broke the inside was a whiteish colour, i sont know what it was and how it got there, anyone got any info??