Medieval Gem Values
I'm working on developing a realistic economy for my setting, but one thing my research has failed at is finding numbers that I can use to create a gem system that can be used to price gems of various quality. I'm aiming for a fairly simple system, even if that deviates a bit from history (though I'm trying to ground it in historical prices).
My goal is to, for a given type of gem, have a formula like:
price = (base price) * (quality) * (size factor) * (cut)
So that means that this question has a few parts to it. In all cases, I'm looking for answers in circa 1300 Europe (late middle ages, before wide external trade soured the market). Gem prices were fairly consistent across Europe at that time according to my own previous research (which didn't provide numbers for this consistent price). I can also convert between European currencies of the time, which were far less volatile than modern currency, so answers in any European currency are fine.
1. What is the relative price of gems compared to each other? I know that the rough order is something like sapphire/ruby > emerald/diamond > amethyst/garnet/citrine > pearls > lesser stones (eg jasper), but how much would, for example, a 1 carat, well-cut, average quality stone have priced at (for at least the first 3 sets)?
2. 878
How does the price change with size? For example, in general, if you double the size of the gem (eg 1 carat to 2 carats), does the price increase by a factor of 2? 3? 10?
3. What would be a good rule of thumb for cuts and quality? I'm thinking something like x1/2 for a dull stone and x2 or x3 for a particularly lustrous or fiery stone. Similarly, I was thinking maybe x1/2 for a poorly cut stone or x1/4 for an uncut stone. Is this reasonable, or are my numbers way off?
I know this is a bit of a long question, but any help with even one portion of one part of it would be greatly appreciated!
EDIT:
To clarify, I'm not looking for answers to "Were gems a major part of the economy?". I'm looking for "If someone had a gem they wanted to sell, how much could they sell it for?"
I've encountered multiple sources (one presented here) that gemstones were widely owned among the upper classes, and during the late Medieval period (which is the subject of this question) it was even a large enough market for forgers to get involved.
There are some sources that also provide some limited price information. The information from these sources is very useful for determining that the price of a ruby could be in the neighborhood of 1 pound sterling (expensive, but not prohibitively so for upper class merchants and above), but without details of what size the stone was, it's hard to know if this is an upper or lower bound. 878