Interesting concept Maha - kind of a simpler variation on my smuggling idea.
I think a craft ingredient that players must fight over in naval combat an interesting idea,
What happens if the ship is sunk. Can gems be recovered by shipwreck hunt
How do we get these gems in the first place?
Problems with your suggestion -
What if I store my 100 gems in ships in my marina?
What if I store my 100 gems in ships that are out of fleet?
What if I store my 100 gems in ships that are stationed at a port and I don't move them so they don't show on gossip,
and I avoid incurring danger on them so they don't show on the plunder board unless I get fugitive cast on me?
Here is a suggestion - gems or alternate ingredient - we will use gems for this example
-Flags on both gossip and plunder board
-You must win the battle to get one or all of the gems in cargo
-Merchant gems - for mercantile voodoo crafting must be charged by being carried on ships delivering cargo - x amount of crates delivered by the
fleet carrying the gems is equal to a full charge
-Pirate gems - for raiding voodoo crafting must be charged by battles, - x amount of battles by the fleet carrying the gems is equal to a full charge.
Edit - adding this note about ransom
Ransom should be per gem - so if you have ten in the fleet - the attacker can take one or any number up to 10 of the gems or the balance in ransom.
Is gem ransom pre paid or pulling from your treasury as this could affect ship ransom if it pulled from your treasury.
Per Bmw - requirement that gems be transported on ships of certain stature in combination of my suggestion that gems need to be charged may boost usage of those ships.