I have a stone, I usually keep it in my oven ~ about 1 hour with 500F. The pizza turned out to be very tasty except the bottom - dry and firm (~6-7 minutes in the oven). What should I check? UPD The base is not just crisp, it's really hard to bite and chew. My dough:  flour 200g water 140ml salt 4g yeast 0.4g  The process:  I mixed water and flour, leaving for 0.5hour Add yeast to a small amount of warm water Add salt and yeast mix into the dough, mix to incorporate Leave for ~40mins Fold Leave for 6 hours. Bake  The recipe is from the "Flour, Water, Salt, Yeast" book. 
I believe your water content is too high. I use a reciepe similar to yours to cause the exact effect you dont like. High water content cause a crust so chewy it is hard to bite through. Try 90-110ml of water instead of 200ml. When the stone hits that high water content is does something similar to when you mist bread in the oven to make blisters. 