My son is principal at a school in n. CA where all of the walking area in a portion of the school is Decomposed Granite. They are going to build some raised beds for the kids for veggies, etc. & want to get rid of the granite since it tracks into the school. I told him I thought it could be used interchangeably for lava sand in the beds so don't throw it away! About the only use they have for it seems to be for walkways. And good amendments are pretty hard to find out there!
I have found the potting soil recipe here which includes granite but also includes lava sand, shale, & green sand. BUT does the ratio vary for a planting bed? And what if the granite is the only rock ingredient that can be obtained?

Chances are they can get some if not all of those other amendments there. A lot of the lava that is used in landscaping and ground for lava sand comes out of Northern Mexico, is transported through Arizona, and out to the U.S. If we can get it here in Texas, surely they also transport it west into California! (Northern California is part of that geologic Pacific Rim of Fire and has a lot of volcanoes, so they probably don't need to import lava sand from anywhere.)

Meanwhile, getting them to do what they can is better than doing nothing. Mix good topsoil with compost and the amendments they CAN get, and work from there, adding each year and as the budget allows.