white rock in texas, is this aquarium safe?

Take some white vineagar and pour a couple of teaspoons on the rock. If it bubbles, it's no good for your tank.

As far as the pH question, I couldn't begin to know unless we made specific ID on the rock. But I don't live too incredibly far from you and I wouldn't trust local rocks in my tanks.

If you like, find a landscaping company who can provide you with aquarium-level rocks for pennies on the pound (and keep you from having to pay LFS prices). I see unbleached holey rock every day on my way to work, sitting in a landerscaper's supply yard.

It appears Texas Holey Rock is safe... is this the rock of which you speak?

If you are looking to supplement the live rock in your tank, you want rock that is very porous. Solid rock, like granite, will not contribute a great deal of surface area for bacteria to colonize, and will reduce the space available for other, more porous rocks. These denser rocks also contribute to the overall weight of the system, which can cause a problem.

Also--having it bubble doesn't necessarily exclude it from a SW system. Carbonates, like crushed coral, help buffer pH to the appropriate level. The acid test is helpful for items going into FW low pH tanks, not so much with high pH tanks--including SW, BW and FW African cichlid tanks.

Texas Holey Rock can be a reasonable and affordable alternative. The only problem is that it's too dense. Texas holey rock can be 5 times heavier than the FIji live rock with the same size. If this doesn't matter to you, go for it! Holey rock has a limited buffering capability. It will not significantly raise the pH level, but help maintain the pH. One warining: Some of Texas holey rocks contain iron (in an oxidized form). You need to be cautious unless you want to see many undesirable brown and green algaes thrive in your tank. Hope this helps.