I would love to give this topic more attention than I have right now to talk about it, but here's a summary of my thoughts:
Valuation of the diamond is impacted on several nonlinear factors I can think of: immediate availability, rate of replenishment, and usefulness (lets call it the ARU score).
Skimming over each of the factors:
Immediate availability of diamonds is a number that will steadily increase for the life of the server map (making the assumption that we'd never mine all the diamonds while still stockpiling and using the mend enchant). So that slowly decreases the value of the diamond.
Rate of replenishment is more complicated, where you have to take into account an average player's effort to get a diamond (diamonds per hour). And worse, this has the most weight in determining the diamond value. This factor plays heaviest into the idea of "supply and demand." In general, the rate of replenishment for the diamond is incredibly low compared to most other ores since all others can also be obtained through smelting, use of iron/gold farms, villager trades, etc.
Lastly, is use. Depending on your playstyle, Iron or emeralds have the highest usability values, meaning you're able to do/get more things out of them. Diamonds are more of a "secondary" use, as you want better tools and gear to get/do more things.
I'm not going to pretend that I understand all the relationships that play into the value of the diamond, but I do know there are a ton of things that have to play into it, and those factors change by the hour. Here's my observations:
- The supply of diamonds is ever increasing due to its low use value
- Despite the increase in supply, it is severely outpaced by iron and emeralds due to villager trades and iron farms
- Gold has the lowest ARU score, and would have been a great alternative to the diamond if it weren't for the fact you can smelt and use gold farms to quickly inflate the supply overnight.
Based off what I understand, here's some of my informal price list:
1:37 diamond to iron, driven by irons high rate of replenishment (my personal exchange rate is more like 1:8 because I basically breathe iron for life).
1:18 diamond to gold, driven by golds low use & immediate availability
1:85 diamond to emerald, driven by villager trades
... and then these values are altered further by the supply and demand of resources when you're actually interested in the trade.