base rock ?????

[pirate]

Well I'm setting up my tank and have a few questions about rock. For the base rock can I use any old rock or is it better to use porus rock? I'm gonna have a sand bed and I'm wanting the rockwork on the glass befor the sand goes in. So does it matter? Can I use rocks from the mountains? Just trying to find something to put under the sand for the base work for the reef.
Thanks