For those people who got shadow crystals during the roll call event.
The crystals affect the end experience and if one person uses it applies to the whole party.
The effects also do not stack with other crystals.
Tested out with guild on what I stated. Save your crystals for high experience yield shadow missions.
They also do not work in castle missions, generation finals, and any of the three underground waterway missions according to the descriptions.(Going to see if it works in golden apple some other day)
Normal Crystal~+80% Experience
Fine Crystal~+100% Experience
Finest Crystal~+120% Experience.
So if it does work in golden apple hard mode you'd get 300,000 + (300,000*1.2) = a whopping 660,000 exp! with a finest shadow crystal.