Final Fantasy XV has a rare and precious stone called the Earth Gemstone. It's far more useful as a crafting tool than being sold for gil in the game.
Earth Gemstone for Upgrading Valiant
The Earth Gemstone is a vital ingredient for upgrading the Valiant which is a weapon that could be obtained from a guy named Tony. Final Fantasy XV players who got the weapon will immediately receive a "quest for a Better Valiant." Player would have to talk to Cid first who will hand out the quest.
Finding the Earth Gemstone
One of the Earth Gemstone can be obtained while taking part in another quest called "Where She Lived." This actually takes place later in the storyline during the part Final Fantasy XV players leave Lucis. The particular quest will begin in Tenebrae where players will talk to Aranea in a concrete bridge.
After talking to her, leave the bridge and head for the plateau where a number of Imperial shuttles are parked. Turned right and proceed to the first shuttle. Players can find an Earth Gemstone lying beside a bunch crates near the entrance. The particular crates are the ones located in the middle of the cliff and shuttle.
Bring Earth Gemstone to Cid
Bring the Earth Gemstone including the Valiant to Cid who might be staying at either Cape Caem or at Hammerhead. However, it will take him several days to upgrade the Valiant weapon. It is highly recommended to take a quest or two while waiting for the crafting to be finished. Final Fantasy XV players may also have to rest or sleep first before Cid will inform you that the upgrade work is finished. So far, Final Fantasy XV players can only find the rare Earth Gemstone in one location. However, there might still be several places where it can be located.