I think they were "ordinary" stones, except probably that they'd surely be prized just as stones. I'm not an expert in these things, but I mean that they would have been probably valued gems among the goldsmiths&co., like in our world there are some prized exemplars of diamonds and whatever (though the only example I can think of now is the Pink Panter
). But aside from that, these stones, I am quite sure, had nothing special on them, at least not any "powers" or similar things.
However, what exactly they were I'm not quite sure - Galadriel's Ring had surely diamond in it, but it would be interesting if anyone could bring any evidence of what the other two stones could have been (though sapphire and ruby are the most typical choices, Alfirin is right to say "presumably", as we don't know for sure).