text: how can i tell if a coin is silver and not some other metal.? i was recently given a sack of old foreign coins from a relative. most are probably worthless but a number of them look like silver. how can i determine if a coin is silver. thanks, roger williams place a small lump of boiled egg yolk against the edge of the coin for several hours or overnight.  the sulfur in the yolk will make the contact point on the coin edge turn dark gray or black (formation of silver sulfide).