A couple of useful links about what rocks are safe to use in aquariums. This is a good descriptive layout that may have been lifted from another website. Alternatively there is also this one which isn't laid out as clearly but still useful. Btw, the only sandstones that need to be avoided are those with >25% feldspar, known as arkoses - they're quite distinctive as they are very coarse-grained as can be seen in this example.