Help please!!!

So my son is almost 19 months old. Today I was sweeping and he was sitting playing I turned to look at him and he was chewing on something so I went and opened his mouth and I'm pretty sure it was a rock. It was dark gray and very hard but a bunch of little pieces were coming off of it. I dug out what I could then had him rinse with water and spit then i brushed his teeth. I'm worried should I take him to the doctor? If it really was a rock would he have been strong enough to chew off pieces of it? It was very small. And he has 12 teeth and he's cutting 4 more. Any advice? I feel so horrible.