Much like the Valentine’s Day “Share the Love!” Offer, there’s a new offer allowing you to purchase gems for yourself, and for all of your fellow alliance mates.
UPDATE (again): this offer has been brought back once more in July 2020. No changes to the offer since last appearance.
UPDATE: the December 2019 instance is the third time an offer of this sort has ever been offered, and the second time for this particular offer. Share the Gems was last seen in July 2019.
@princess1 wrote a message you can copy/paste into alliance chat to thank purchasers without pressuring those who prefer not to spend money:
When an alliance member purchases the offer, this message shows in the alliance chat (I’ve blacked out the player’s name):
When you visit your Inbox, you’ll have a message with gems for each alliance mate who’s purchased the offer: