What are Power Gems used for?

//ads -- adsterra.com -- native banner
?>
I've found several power gems so far -- the description doesn't say anything about what they are used for, or how to use/activate them. What are they used for?
This is what they look like in the inventory:
Also in the "Stats" window of the main menu, it says "Power Gems Found: 3/20"
What are these things?
Pictures about "What are Power Gems used for?"



What does the Pokemon move power gem do?
Power Gem is a Rock-type Main move in Pok\xe9mon GO that deals 80 damage and costs 50 energy. It is strong against Flying, Bug, Fire and Ice Pok\xe9mon and weak against Fighting, Ground and Steel Pok\xe9mon.What Pokemon can learn Power Gem?
Known Pok\xe9mon that can learn Power Gem naturally include Ampharos, Boldore, Carbink, Dialga, Diancie, Flaaffy, Gigalith, Grumpig, Johtonian Corsola, Lunatone, Mareep, Minior, Misdreavus, Mismagius, Necrozma, Nihilego, Nosepass, Palkia, Persian, Probopass, Roggenrola, Sableye, Slowking, Spoink, Staryu and Vespiquen.What type is power gem?
Power Gem (move)TypeRockCategorySpecialPP20 (max. 32)Power80Accuracy100%2 more rowsWhat do power gems do in Phantom Hourglass?
Below is a listing of all 20 Power Gems that Link finds on his quest. When Link has collected 10 Power Gems he can restore a portion of Leaf's power. In order to do so, he must take the gems to Spirit Island, which is a hidden Island located just south of Molida Island.What ARE the Gem Species in Steven Universe?
Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.
Images: Pok Rie, Emilie Leenaerts, Andrea Piacquadio, Anete Lusina