Stop me if you've heard this one... Which weighs more: a pound of gold or a pound of feathers?

No, they don't weigh the same.

And no, it is not because of buoyancy.

Gold, like all precious metals or gemstones, are measured in Troy Weight (which is named after Troyes, France). The basic unit of measure in Troy weight is the troy ounce (abbreviated ozt), which is 480 grains (a grain [abbr. gr] is a unit of weight that is exactly 0.06479891 grams). The ounce that we use to measure pretty much everything else is the avoirdupois (Anglo-French for "goods of weight") ounce (abbreviated oz), which weighs 437.5 grains. So a troy ounce is about 10 % heavier.

So now you think that a pound of gold is heavier, eh? Wrong! The problem is that there are only 12 troy ounces in a troy pound (abbr. troy), whereas there are 16 avoirdupois ounces in the avoirdupois pound (lb). This means that

1 troy = 5760 gr < 7000 gr = 1 lb

So, in fact, one pound of feathers does weigh more than a pound of gold.

The Troy weight also has the other significance as the reason Britain had that wacked up coinage system before decimalisation. The penny in British coinage was literally one pennyweight (=24 gr) of silver, while a pound sterling, which is worth 240 pennies, is exactly 1 troy of sterling silver.