is very very porus. Is there a reason you want to seal the stone? If you have one, I would then contact a nursery or stone retail outlet and find out from them. Everything I can think of would weather off especially from rocks.
There is a silicon based product that costal cities spray on bridges and other structures near the salt destroying air. I know for sure that Corpus Christi, Texas Department of Streets does this. Its a silicon spray that is applied to the concrete, brick, steel girders, and other outside stuff. You might contact an agency like this for more information. This spray was to counter the negative affect of salt air on outdoor stuff.