I was looking in ebay for AG (as you do) and there are heaps of listings for a Silver Purity Checker Kits with mystery 1/2 ounce bottle of Silver test acid along with a testing stone for the low price of one ounce of silver 29.95! I wonder how effective they are or they selling us cheap minerals we can pick up at the hardware store? Does anyone own one? How reliable are they? Stories on modern and ancient purity checkers welcome. Very curious.