Of currently somewhat active shops according to forum posts:
No shops currently have two-way exchanges. (IE if I have 64x I can get 1y out of the shopkeeper, where the price is 1y for 64x.)

Gold:
Haxington Oreburgh Mine: 8 ingots = 1 diamond
Andureil: ~14 ingots = 1 diamond
Ealdwine: 64 ingots = 1 diamond

Iron:
Haxington: 16 ingots = 1 diamond
Ealdwine: 64 iron = 1 diamond

Diamond:
Everyone: 1 diamond = 1 diamond
Everyone: 9 diamonds = 1 diamond block

Emeralds:
Ryebye: 1 Iron Horse Armor = 3 Emeralds
Ryebye: 1 Gold Horse Armor = 2 Emeralds
Andureil: Knockback 2 Book = 5 Emeralds

Cobblestone:
Haxington: 256 (4 stacks) = 1 diamond
Ryebye: 320 (5 stacks) = 1 diamond
Ealdwine: 384 (6 stacks) = 1 diamond

The first numbers listed are shops that have not recently been updated for a while. The last numbers are the shops that have been recently updated.

Right. But diamonds are becoming increasingly more valuable too quickly.

Also, Ryebye just bought all of Ealdwine's gold. kek

That's right. We have the opposite problem.

Simply put: People aren't spending diamonds.

The thing is, at this rate, it may cost a ridiculous amount or resources to buy one single diamond.

Just as inflation is bad, deflation is bad as well.

The thing is, if diamonds are to be the standard currency, they are too valuable for being a standard currency for what they can do.

It's not clear. I'm guessing @Cellexya is the richest atm.

I don't mine much and I have around 20 total diamond gems.