its heavier than alumWelcome to Tnet
Platinum is around eight times the density of aluminium, twice the density of silver and almost twice the density of lead, so you wouldn't have too much trouble making an 'in-hand' judgement about whether it's platinum, aluminium or something with a density between the two.
You haven't told us anything about its properties. Is it attracted to a magnet and, if so, how strongly; was it tarnished when you found it (ie have you cleaned it); how hard is it (can you scratch it with a pocket-knife, piece of window glass, 'copper' coin etc)?
A locality would also help in determining whether naturally-ocurring native metals are likely to be found in that area (versus something man-made).
melted aluminum in every rural stream i hunt in? hmmm maybe...i need a X-Ray gun out in the fieldIt looks like melted aluminum.