Exchange ratio among various gems?

I know Perfect Amethyst has the highest value among all kinds of gems, Perfect Ruby and Perfect Skull also have higher values, but what should exchange ratios among different kinds of Perfect Gems be.

I am thinking something like:

1 Perfect Amethyst = 3 misc Perfect Gems ( Perfect Diamond, Perfect Topaz, Perfect Emerald and Perfect Sapphire)
1 Perfect Ruby = 2 misc Perfect Gems
1 Perfect Skull = 2 misc Perfect Gems

Amethyst 3x I agree
Skulls 1 - 1.5x
Ruby 1 - 1.25x

I feel like everytime I look at pgem sales, the Amethysts are gone, and the seller is trying to get a premium on Perfect Ruby/ Perfect Skull but no one ever buys them.

Being a renowned expert on the subject, I'd say Perfect Amethyst are worth as much as any other perfect gem... So I'm ready to offer 2 Perfect Gems for every perfect amethysts you guys have!

It's widely agreed that P-ameths are the most valuable gems of them all.
For me personally though, I place topazes in the same value category as rubies. Topazes are great for socketing into head or body armor to increase MF (only 1% behind socketing with Ist rune), plus topazs are involved with the socket recipe to turn a White armor into a 1 to max socket runeword base, which I like to use alot. So I personally value gems at:
1. Amethysts
2. Rubies & Topazs
3. Diamonds, Sapphires
4. Emeralds
5. Skulls

For the purposes of trading though, it really depends on the buyer/seller when it comes to non-Amethysts, YMMV.