The Faberge Egg is an unseen artifact housed inside the Library. Another Fabergé Egg, apparently worth $35 million, is seen being deposited in the Bank of Thieves in "And the Christmas Thief".
Powers[]
As it was never seen on the series, its powers are unknown.
Real Life Counterpart[]
Fabergé Eggs are extremely expensive, egg-themed jewels, made with precious materials. The most famous of them were made in the late 19th Century and, since then, having one of those is considered a huge symbol of status.