If you have a good scale measuring to 0.1 gram or so and a small
container full of water, play Archimedes. Note: The container must
be large enough to allow complete immersion of the objects.
Weigh the questionable item and put it into the container of water -
the displaced water will overflow. Carefully remove the item, being
sure to shake any beads of water back into the container. (You might
want to use a hook.)
Weigh out some gold (14K is commonly used if you’re in the US) to
the same weight as the unknown; place the gold into the same
container and see if the water level comes up to match the same level
as when the unknown was immersed. If so, the material of both objects
is the same.
Actually, the order of unknown or known first doesn’t matter. You
are concerned with whether or not the amount of water displaced is
The denser material (gold, Pt) will displace less water than the
same weight of a less dense material (brass, silver, etc).
Google Archimedes Principle for more info.
Judy in Kansas, where the forced branches of forsythia are beginning to
bloom - “Oh frabjous day! Callou! Callay!”