Out of curiosity I looked at the Quikcrete MSDS for the patio base sand, they only categorize it as "CALCAREOUS OR SILICIOUS AGGREGATES FOR USE IN CONSTRUCTION" in the PDF. Unfortunately that PDF seems to cover all of their sand products.
Under the hazardous materials section they do list limestone in addition to quartz. You could test by some vinegar to see if it bubbles, or HCL, some form of acid.
It's probably safe, but last time I saw a bag it was seriously dirty, lots of rinsing would probably be necessary. I suspect they'd need to list any additives if they were dangerous. Good rinsing should eliminate any soluble items. I'd probably pass though if the purpose was just for something inexpensive. I'd recommend the play sand for certain in that case.
I am on the look out for a sort of larger grain multi shade sand, those various construction grades are probably good for that, whereas play sand tends to be either white or beige.