The Jewel of Fawton was the hive mind that connected each of Agratis' crystalline creatures' thoughts and nervous systems.
When one species colonised Agratis, they took the jewel while the creatures were hibernating, and based their entire society and advances in technology around it. When the Jewel became a museum exhibit, the Fourth Doctor said it was spoken of as "the most beautiful gem in all of creation" and that some said "you [could] see infinite complexity in the facets of the Jewel".
Roget, discovering its true purpose, returned the Jewel, reawakening the creatures. The Doctor realised that the colonists had to find a way to live with the crystalline creatures and no longer relied on the Jewel as a source of income if they wished to stay on Agratis. (COMIC: A Rare Gem)