I think you've reversed cause and effect here. Gold reserves prove that people use it as a store of value, but they're not a necessary condition for something being a store of value.
For example, let's suppose that I made a deal with Porche to buy the first car off their assembly line each year. I take it and store it away as an investment. Is there an existing reserve of 2018 Porches? No, I have the first one. Does anybody else do this? Doesn't matter. It's still a good store of value as long as people keep wanting 2018 Porches.
People could use steel as a store of value; it's just an inconvenient commodity to deal with because the value per gram is so low. You need to store a lot of it. But rarer metals will do fine.
