Kinematics
Hero
As for the value of the diamond, I think that's more of a meta-rule than an in-universe rule. It's a rule for the player, not the character, because what it's really there for is to be a resource sink to use up and stress the gold supply of the characters, which is what all wealth translates to.
The rules could have said, "This spell requires a 2 carat diamond, which is consumed on use", but then the question becomes, "How much does a 2 carat diamond cost? How many of them can I get? Do I need a gemcrafter's toolkit to appraise the diamonds?", and then you need to put in a rule set for scaling gem sizes to gold costs (for all types of gems), and it's just a lot of extra overhead for no real gain. Instead, you just have a 50 GP onyx, or a 500 GP topaz, etc.
... Hey, did you know that diamonds aren't listed in the 5E DM's guide's gem tables, in the 100 GP – 1000 GP range? The only one listed is for 5000 GP. Curious.
Personally, I'd guess that the 300 GP/500 GP/1000 GP costs for the spells really translate to 1 carat/2 carats/3 carats (it's a non-linear curve). Once you do it that way, then the Ragnar scam doesn't really work (though a scam with cubic zirconia would likely still be possible, but without it still working for spells).
The rules could have said, "This spell requires a 2 carat diamond, which is consumed on use", but then the question becomes, "How much does a 2 carat diamond cost? How many of them can I get? Do I need a gemcrafter's toolkit to appraise the diamonds?", and then you need to put in a rule set for scaling gem sizes to gold costs (for all types of gems), and it's just a lot of extra overhead for no real gain. Instead, you just have a 50 GP onyx, or a 500 GP topaz, etc.
... Hey, did you know that diamonds aren't listed in the 5E DM's guide's gem tables, in the 100 GP – 1000 GP range? The only one listed is for 5000 GP. Curious.
Personally, I'd guess that the 300 GP/500 GP/1000 GP costs for the spells really translate to 1 carat/2 carats/3 carats (it's a non-linear curve). Once you do it that way, then the Ragnar scam doesn't really work (though a scam with cubic zirconia would likely still be possible, but without it still working for spells).