The 7D's jewel mine is a huge mine beneath their home that has many caves full of jewels that the 7D dig up.
The jewel mine is huge. Beyond the main area that has lots of jewels, the mine has a set of tracks that lead through a variety of terrain and pass many rooms.
The jewel mine is known for its large amount of jewels. The 7D's main job, when they're not helping Queen Delightful, is harvesting jewels from the mine. The mine has many notable features such as a garbage recycling system where trash is transported through a water system that leads into a recycling machine. More notably, the mines are home to the Rock of Sages which is a gem of immense power and it's guarded by a dragon in one of the most exclusive sections of the mine. The mine is so important that the 7D have to keep its entrances a secret.
In the series
The 7D can regularly be seen harvesting jewels from the mine prior to being summoned by Queen Delightful. One of the Glooms' main goals is to gain access to the 7D's jewel mine as it serves as their motivation for many of their diabolical schemes. This fact and mines play a very large role in "Welcome to the Neighborhood" where the Glooms disguise themselves as small dwarfs to gain access to the mines and with the ultimate goal of stealing the Rock of Sages.