As of March 31, 2021 12pm JST, jewels can no longer be purchased in the game or through GASH. This is due to the game shutting down at the end of May.
There are many uses for jewels, but not all of them are worth the price. A list of all ways to spend jewels, from best to worst:
- Increasing Deck Cost.
- Buying Card Limit +5 for  Jewels each (maximum 90).
- Upgrading your Castle past lv9 ( for lv10,  for lv11 and up).
- Upgrading certain buildings to their max levels.
- Unlocking additional pages of your displayable collection for  Jewels per page (maximum 70).
- Performing Alliance Bingo Battle summon for  Jewels.
- Buying certain recovery items (Valkyrie Rods and Crystals, Phoenix Feathers).
- Buying jewel-exclusive decorative buildings.
- Performing premium summons (summon for  Jewels each or  Jewels for 10+1 premium cards).
- Finish buildings in an instant.
- Recovering 1 entry in Elemental Chronicle.
- Buying resources for one of the 4 resources types. You can choose from 10%, 50% and 100%.
- Recovering your cards without using any resources.
- Resetting the duel timer to 00:00.
Obtaining Free Jewels
Jewels can be obtained from area rewards, quest rewards, login bonuses, during World of the Gods Slots events (as a prize of 5 or 100), Collaboration Rewards, Tapjoy rewards (in Summon page under Free Jewels!), Holy Token Summon, or sometimes as gifts from the devs. They cannot be found from advancing through areas and Archwitches do not hold jewels as rewards.
It is important to carefully manage your free jewels. It is not recommended to spend free jewels on summons early in the game, since the rate of obtaining a useful SR or UR card is very low.
The current prices for jewels (taken from the official in-app prices) are:
You can also purchase jewels for yourself or others using the GASH system on the "Gash Point" website. The purchase page used to be on the old Nubee website, but recently moved directly to the Gash site. The price per jewel is slightly less than purchasing directly in game.