They are incredibly valuable on Earth and it is said they can be sold for enough money to last ten life times.
Use by Cêpan on EarthEdit
Despite being extremely rare and worth a lot of money, Adelina (Marina's Cêpan) travelled through Europe and could not sell them, instead she was forced to trade them for hot meals and they barely scraped through until they were taken in by the Santa Teresa convent. 
It is unknown exactly how the other Cêpan such as Hilde, Conrad, Katarina, Reynolds and Rey dealt with their gems but it is likely that they were sensible and used their money to stay on the move, buying things that they needed to train and live.
On the other hand, Sandor used the riches from the gems and settled down in a penthouse in the John Hancock Center, here he and Number Nine lived in luxury and enjoyed the best of everything, including cars and state of the art training facilities (Lecturn) and weapons. Sandor was also in the financial position to explore his hobby of inventing electronic gadgets. 
Marina commits the gems to the Loric Sanctuary in Mexico to jump start Lorien. It is unclear if they saved some for future use.