Thw best sand for ballast dampening is?

I am buying a sandbox amp stand for preamp. My only experience is using steel coated lead shot. I was given the shot for VR2s which have a sealed off compartment for that purpose.
I have heard that there are very dense shot like materials that are better than either fIne clean sand or lead shot.
Anyone know what to use and where to find it. I am curious about this new material I think it is a black color metal alloy.
This might be what you seek Microbearing
I also think this is what you have in mind. Add it in small amounts as it is VERY effective and you can over-damp the system, more is not necessarily better in this case.
Need to check your periodic table of elements, which I am sure you quickly gave away when your last chemistry class was over.