Throughout Magic: The Gathering history, few groups of cards have remained as powerful as moxen. These jewelry-themed cards could do a lot based on color.
The first five (Mox Emerald, Mox Jet, Mox Sapphire, Mox Ruby, and Mox Pearl) were each zero-cost artifacts that tapped for a specific color of mana each. And they comprise the majority of the power nine. Expansions in the following decades provided updated spins on the original moxen and somewhat nerfed them, but they continued on. Even Magic's Un-sets and Magic Online got some.
But just like Magic's oft-rumored sixth color (purple), there was an original sixth mox.
In 1996, a few years after Magic's debut, plans were made for a new mox card for the upcoming Mirage expansion. You know, THE one mox bring together the five colors of Magic.
So a card known as Mox Crystal (or Crystalline Mox) was created for playtesting. According to the playtest rules text, Crystalline Mox would be an artifact that would cancel out any other Crystalline Mox in play (meaning you could only have one out at any one time), and, when tapped, adds one colorless mana.
However, when tested, it didn't do so hot. The exact reasons have never been disclosed, but it just wasn't working. However, there was a slight problem - they had already commissioned some kickass artwork by Dan Frazier. So there had to be some kind of solution. It was either keep in the mox that caused problems when playing, waste some great art, or....a third solution.
Not surprisingly, they went with number three.
"I was commissioned to do the artwork for the card 'Crystal Mox," said Frazier in an interview. "When I spoke to Sandra [Everingham], I said, 'Oh, that's nice, since I've done the art for all the other Moxes.' She responded: 'You did the other Moxes?' Back then, when the game was still new, the art directors weren't as involved in the rest of the process. They never printed Crystal Mox, but the artwork that I did for the card is the artwork on Enlightened Tutor. You can kind of see what I was going for if you noticed the jewels of each color in the Tutor's crown."
Yup, Magic being Magic, Wizards of the Coast reused that artwork for another card.
To date, there has not been a Mox Crystal put in in the ensuing 25 plus years since Mirage, though (unofficially speaking) a Mox Crystal did get printed as a promo card for the casual "5-Color" format.
WotC doesn't always release details of cancelled Magic cards, but when they do, the story is almost always a doozy.