Unique Gems are gems that can only be won during challenges or stolen from the winner of a challenge. They have a pretty high rating and look fantastic. After a few challenges, the Unique Gem at stake will be changed, and you won't be able to win the previous one in future challenges.
The trick is, once a Unique Gem is stolen, it cannot be retrieved by you or your friends/guildmates. Thankfully, if you put Unique Gem into the totem, it will be disguised as a simple gem. This way, if someone attacks you, they won't be able to distinguish the Unique Gem from the other ones.
Starting a ritual with a Unique Gem in it will not only lead to you receiving a new, more valuable Unique Gem, but will also give you special reward depending on ability of a gem you placed in the totem.
There are three unique gems and two special abilities.
Handful of blue orbs
Unique gems with this ability will give you blue orbs if the ritual is finished successfully. There are two unique gems with this ability: a Feather Gem and a Tinkling Gem.
Teeth recovery
The second ability will keep your totem safe. If you finish the ritual with this gem installed, a tooth in your totem won't be destroyed. For now the only gem with this ability is a Snow globe.
No ability
There is one gem with no special ability. But this gem was available only during one single challenge.
Unique Gems also won't be automatically sold if they are the least valuable ones in the inventory, and manually selling them will give you orbs as well as gold.