The following info was taken from www.audiopoints.com:
Recommended Fill Materials
Good - White Silica--a pulverized dry glass that will add density to the tubes. NO SAND - too absorptive. Glass is a non-conductive material that does not absorb tremendous amounts of energy. Silica has a very consistent grain and texture, as it will increase mass and stability for heavier components.
Better - Steel Shot--the smaller the size of shot, the better. Steel, like our shelving, conducts energy vibration and adds speed to the grounding pathway. NO LEAD, as lead is a primary energy absorbent material and will reduce the rapid transfer of energy through the pathway. Check with your local gun shop.
Best - Crushed Iron Ore--adds incredible conductive power to the main support rods. Both rods react as one, creating a tremendous high-speed pathway to earth's ground. Crushed iron ore is difficult to find; check with mining and steel manufacturing companies.