Here is another test: take some sand and put it on a piece of glass. Rub it hard with your finger, if it scratches the glass it is silica. I use a brand of "Play Sand" that Walmart carries after washing it well, and it serves. As we know sand is not just sand when it comes to CP, so you want to be sure of which "Play Sand" you really have before potting a lot pf plants in it.
Another good source for silica sand are businesses that deal in pool supplies: silica sand is used in pool filters, and is coarser than play sand, and I prefer this product.
I believe this is also stocked by Walmart.