Hope to god people are not buying this for the Loadstone quest. You can farm a meteorite at Starfall Hillock in West Sarutabaruta off the crawlers with ease. It's free and it's a fairly common drop, save yourself the gil.
Skip the Gems and Geodes for the Open Sesame Quest.
average price should be ~10% soil geode x99 price.
A stack of geodes completes 8 loadstone quests, so that's really all it can sell for. any less and it doesn't pay the cost of the crystals & geodes, any more and geode is the better option for completing the quest.
