I’ll vouch for the instability and potentially explosive quality of picric acid — and I have my own hospital records to prove it. Picric, however, is a light lemon yellow color and granular, so it doesn’t correspond to the description of a flaky white residue. Nevertheless, your comment underscores the kinds of danger that can be posed by unknown chemical/pharmaceutical substances. Most are benign; some are anything but. Satisfying one’s intellectual curiosity about such stuff may not be worth the cost.