Well, assuming the Hero can end up with any spellsilver in the first place.
Lustrous, silvery, ductile, soft and easy to manipulate. It'd have tarnished like silver, but faster, visibly so by the time he'd even left the dungeon. A centimeter cube of it would be gone completely in a year, turned to a grimy dirt, faster if you were exposing more of it to the air. If you get a little powder of it it'll outright ignite in air.
Almys hadn't quite repeated the claim of a centimeter cube being gone completely in a year; just claimed he'd expect it to take about that long by eyeballing it.
The store's proprietor, a middle-aged woman with gratuitously large endowments wrapped in tight gappy clothing, seems to think this description potentially covers a number of metals. Her first guesses are Substances 11, 19 or 37, as all lie apparently in the same column of some sort of Chart of Alchemical Wisdom. They're all soft, silvery-white metals that will tarnish in air! And explode in water. Did the customer try reacting any of his dungeon-drop with water, in the course of whatever mysterious experiments have him so excited? Did he observe the melting point in morrisons, perhaps?