Jal rock

I think that jal is volcanic stone. It is used in construction where a lighter weight material than gravel is needed. For instance, my builders used jal to mix the concrete instead of gravel when they poured the roof over the brick boveda ceilings.

It is also used in landscaping. It wouldn't be a soil amendment unless it is crushed, because most jal is the size of largish gravel and won't break down, but just sit there as a rock.

You should be able to get it at any place that sells building materials like gravel and sand or in smaller amounts at a nursery.