Oh boy, now HERE's something pretty cool!
When Cryptozoic makes trading card sets, they guarantee the declared scarcity of specific cards by printing all the cards they're going to print and then releasing the actual PRINTING PLATES--the metal apparatus used to stamp the design on the card--into the wild.
So each card that exists has four metal printing plates--one in each color for classic printing. It's CMYK--Cyan, Magenta, Yellow, and Black.
Each color card is one of a kind! So they're extremely rare!

They're metallic! You can tap on them with your fingernail and they clang.
When I was buying boxes of SU Trading Cards, I was not lucky enough to find one. And I wasn't too worried about it since it is not possible to get a complete set of any of these. But later, when I was expanding my idea of what a complete collection should look like, I decided to try to get one printing plate in each color by buying them from other collectors.
I finally got one of each color! So I will show you.
CYAN
For the Cyan (blue) card, I got card #42, Taken Away.

MAGENTA
For the Magenta (pink) card, I got card #35, An Old Friend.

YELLOW
For the Yellow card I got card #54, A Strange Dream.

BLACK
For the Black card I got card #48, I Am My Mom.

Here's the set!

I think it's funny that my Blue card was Blue Diamond and my Yellow card was Yellow Diamond, don't you?
Too bad I didn't find a Pink Diamond card (like card #69, Beautifully Flawed) for Black just to be funny. These are the printing plates that created all the cards, so it seems like it would've been cool to get the designs depicting the Diamonds who made all the Gems! But that's impractical since there is only ONE single solitary card of these out there in the world and it's very hard to find printing plates, much less a particular design of a particular color!
Here's how they look in my binder. I couldn't figure out a more logical way to put the plates and example finished cards in a 9-slot sheet.

Here are better detailed images of the printing plates.




Cool huh! If you collect the cards, have you ever found a printing plate?
Comments (3)
The bismuth one looks cool
In fairness Bismuth always looks mega cool
Reply to: swankivy
I agree with that statement.