The earth’s crust is about 25 percent silicon. Sand made out of quartz like desert sand is about 50 percent silicon. Beach sand is usually mainly calcium carbonate from shells and it doesn’t contain much silicon at all. Volcanic beach sand is more likely the same as the earth’s crust so 25-50 percent.
So as long as you refine your sand/gravel/rocks/lava so that you’re left with pretty much pure silicon, you’re good to go.