Is your big rock light, as in lighter in weight than you would expect? If so, I THINK it is a calcite geode. They occur in a couple areas of Texas, 45 miles north of San Antonio and near you, both in a narrow younger formation. Thunder egg, I think that they contain quartz, agate, stuff like that. There is none around you. Might be limestone concretion if it is really heavy. Cool find anyway. Depends where you found it. You can just take a geology hammer and open it to find out. I have found them before, but none that big. It is really nice.