The Sonic Universe is full of gems: Chaos Emeralds, Sol Emeralds, Chaos Rings etc etc. Which ones should be canon and which shouldn't.
The main and most well-known gem from the series.
The firey counterpart to the energy-centric Emeralds.
Strange rings enfused with Chaos energy.
A special Emerald capable of controlling the other 7.
(for those who don't know they're basically the anti-dimensional versions of the Chaos Emeralds from the Archie canon)
Four gems that hold different purposes.
Seven gemstones that reside on Little Planet. They're capable of giving the wielder varying control over Little Planet's time period as well as stabilizing the planet.