Maybe... how big or small are they compared to your tortoise's head? The problem you run into using pebbles is some tortoises will try to eat them. Trying is not so bad, but when they do actually eat them troubles can happen (even death). So I would really really suggest NOT to take the chance.
Darn, I hit the send button before I was ready to.
I would also never use it for the entire substrate, but for a small area it's okay (if the right size). Still like I said, not something I would suggest you to do.
How big are the pebbles ? If they are small enough to fit in your torts mouth then do not use it because they will eat them and get constipated and like someone stated before , it can lead to death . I have my baby tort on coconut fiber which is the same thing is coconut coir . There is a variety of different substrates just do a little research to find out whats best for your Greek tortoise because all torts are different .