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?

1 Answer

Power Gems are used to craft the 4 elemental essences, which are necessary to enter the tower.

You can craft with them by placing 4 power gems on an elemental altar. There's one of these in each major area of the island.

16 gems are required for the normal ending, 20 for the best ending.