Fine grade cube shaped crystals of pyrite. The brassy yellow colour that gives this mineral a similar appearance to gold is the reason why it's also called fool's gold. Some say the nickname came about because pyrite was confused for gold by the non mining community during the great American gold rush. Having looked into this we can find no factual evidence to support it.
Despite a vague similarity in the colour of these two minerals, there are several easy ways to distinguish one from the other. I say "vague similarity" because although gold and pyrite both have a brassy yellow colour and metallic lustre, gold is much more yellow than pyrite. It's really easy to see the difference when the two are placed side by side.
Pyrite is also much harder and unlike gold is brittle. Strike a piece of gold with a hammer and will flatten but do the same to pyrite and it will break into many pieces.
The mineral pyrite is a naturally occurring solid with a crystalline structure which means it's made up of crystals. These can grow in several different shapes but one of the most common is a cube. Isometric or cube shaped crystals are one of the simplest and most common shapes found in minerals.
These fine grade pyrite cubes are being sold individually. Weights and sizes can be found in the adjacent dropdown list. The cube on the left in both photos is number 1.