Shattering

Shattering events are how magic crystals  have appeared in the universe. These events happen through the Milky Way Galaxy's Orion Arm. They are assumed to happen all over the universe, as well. They happen at different times, places, and intensities. They may happen to a world many times, or not at all. Even if they never happen on a specific world, a world could have tiny crystals as part of its formation, as an inheritance of sorts (just as with all other matter it is composed of).
In these events, magic crystals land on a world, spontaneously grow, or both. This may or may not cause destruction, depending on the specific event. It appears that if an even happens on an inhabited world, the crystals concentrate away from heavily populated areas.
Shattering also refers to the other six colors being "Shattered" from white. This was directly experienced when the Alixhemni got their colored crystals originally. As some of the new crystals fell on other crystals (both sets were white), the crystals broke. The results had the other six colors among them. When the Alixhemni began observing other worlds more directly, they saw it happening there too. The other cultures of the Planetary Coalition  have also observed this. 
Shattering events don't always produce the other six colors. Some worlds report that only a selection of colors fell or grew on their worlds. Others report only seeing selected colors break from their white crystals.