The cannabis grower I'm consulting uses growstones in their hydroponic system. I though they were really cool. I've learned that they are not rocks at all. They are actually man made from recycled glass. They are extremely light weight. Even lifting a 5 gallon bucket, I got a pleasant surprise how light the bucket is. They are so light that a few of the stones will actually float in water. I suspect the low density could help move the rocks while in water to allow roots push through the gaps. They are very porous, and due to the open cell structure made of glass, they have a very rough texture like sandpaper or a pumice stone.
I don't know the process how the growstones are manufactured, but I can guess. From the internet, I have found that that the majority of the stone is manufactured from recycled glass. About 1.25 % of the material in the growstone is calcium carbonate. This is what makes the molten glass matrix foam. The recycled glass is crushed to a powder. At some point calcium carbonate is added and the mix goes into a furnace. One thing to note about these growstones is that due to the calcium carbonate, the PH of the water will change when they are first introduced. A water flush usually stabilizes the PH.
Since there are no jagged edges on the growstone, I presume that the manufacturer breaks the solidified foam glass into smaller pieces. From their I would expect that they separate the broken pieces into specific sizes. At some point they must go into a tumbler to smooth out the edges.
Here are some pictures of growstones after they were removed from a hydroponic system. The grower has no intention and considered buying new growstones for each grow an insurance policy to prevent bacteria from invading the hydroponic system. I took some close up pictures to show the texture of the stone. It looks very much like a white volcanic pumice with exception that the pore size is much more consistent than natural rock. Roots separate very easily from the stones once the root ball is left out to dry for a week.
In the following pictures, you can see the tiny roots still on or in the growstone.
I thought that it was wasteful to just throw these stones out after each grow. The manufacturer suggests to steam the stones to sterilize them. The only problem is that wouldn't remove the root material from the stones which would inevitably decompose in they hydroponic system. I thought about using acid or baking them at high temperature to remove the roots. In the end, the cost of the growstones are so low, there would be little cost benefit to attempt to reuse them.
