Sand works very well when used with a quality shelf. You should use play sand. It's very dry and has been filtered. Sand will absorb moisture so overtime you will need to replace the sand. That's not really a problem since it's dirt cheap, but it can get messy.
In the sand box
I have 2 audio buddies who have built component sandboxes with different types of sand. They claim Beach sand is the most dense and made a huge difference when the turntable and amp were set in them. Has anyone else been down this road? I mean there are several ways to isolate equipment, this is just one of many.