I've picked up quite a few of them and they sell for a decent amount, which leads me to believe they are at least semi-important. However, according to the wikia page on them, they're only used in a single quest. That's it.

Is there really no other use for them outside of this quest besides selling them?

2 Answers

Earth gemstones can be used as a magic crafting catalyst to create quad-cast spells with one additional cast per stone used. This is fairly useful for magic casting builds or needs.

As Shawn points out, it is also used in a Cid side quest to upgrade the Valiant machine weapon.

They're used to upgrade a weapon, specifically the Valiant.

However they're also good for making spells, causing "intense" damage and range.