Geode

Location
Geode is hidden within the lower left part of Atteka Inlet where the boulder is, and to have it appear you need to cast the Cyclone Psynergy - but to get to it, you need the Lift Psynergy, which you only get as part of the inventory of Isaac's party when they join Felix's party late in The Lost Age. In Geode's case in particular, though, it can be acquired very shortly after the reunion takes place because the game's plot progression dictates you go from Contigo to Atteka Inlet right away. So, before going to the screen where your ship is, fetch Geode in the pictured area.

Description
When Set, it increases base HP by 12 and base Attack by 6.

When Geode's battle effect is used, the entire background suddenly rolls itself up into a 3D-sphere, and a holographic image of a Venus Djinni propels it into the target, where the sphere detonates in a humongous, 3D explosion before the background reappears. It deals a Venus-based attack equal in power to the user's normal physical attack increased by 90% - the highest consistent damage multiplier of all Djinn, and much greater than the damage multipliers of its otherwise similar counterparts Flint and Echo.

Ability analysis
Geode is one of the most well-known and often used Djinn in the game, both for extremely strong yet reliable damage output and for the elaborate visual attached to it. It should be used by a Venus Adept for optimized damage; however, don't have it attached to the Venus Adept equipped with the Sol Blade easily found at the end of the game, for that deals a consistent damage multiplier of x3 everytime it is Unleashed, and the chance it can Unleash its effect when the user physically attacks can easily be optimized by equipment such as the Valkyrie Mail and Hyper Boots. Geode would never be used by an Adept that can deal a far stronger Megiddo unleash every turn, so it should instead go to the other Venus Adept where it can allow that Adept to deal bursts of damage close to the Sol Blade-wielder.

Name Origin
A Geode is a rock found deep within the earth.