Hi amylama, I had this problem a while ago.
What I did was drill a bunch 1/4in. holes in the
bottom of a five gallon bucket, poured the
rocks in and ran the hose through it for a
while shaking it a little. I then made up
a few buckets of hot water and five precent
bleach solution, poured them through then
rinsed with the hose again. Hope this helps.

I used a big plastic rubbermaid tote to soak my lava rocks in. Left them in there over night to soften all the grime. Then used a clean shovel to get them out and onto my rinse strainer. I made the strainer out of lumber, screen, and hardware cloth. The lumber is nailed together to make a big square 3'x3'. The hardware cloth is nailed across it. The screen is put over the HC. The HC makes it strong enough to hold the rocks, the screen keeps the rocks in place. (i used small lava) I put the strainer on cinder blocks, threw the rocks in it and sprayed it with garden hose.

How much rock are we talking about? Here's my suggestion. Fill a large garbage can with dirty rock. Fill can full of water. Add 1 cup of bleach. Wait one day. Empty water out. Fill with fresh water (no bleach). Wait one day. Strain rock while spraying with garden hose.

If possible find a large plastic garbage can you can fit another can into to use as a sleeve. Drill holes in the bottom of the sleeve. Then place a spigot for a garden hose on the bottom of the outside can, using silicone glue, some kind of gasket or rubber grommet and one of those plastic hot water heater valves. This contraption helps enormously for rinsing large quantities.