Warrior of Lights and Crystals of Light

[Beddict]

We have seen a crystal of darkness though or atleast we have with the Uldah Story. It's right after we defeat Batraal's ugly albinoesque cousin after we fight with the Sultan Sworn with idiots over the stolen Crown. We defeat the black masked Ascian and then noticed a blackish purple crystal on the ground, after we kneel down to look at it it vanishes as if destroyed and has a black purple smoke on it, like that purple blackness that's associated with the Ascians.

Same deal happens in the Gridanian storyline, and presumably the Limsa storyline but I don't feel like looking up the cutscenes on Youtube. Another thing worth keeping in mind is what Urianger says in the 3.4 MSQ "One Life for One World":

Urianger: Just so. As the Ascians flee unto the rift 'twixt planes with Crystals of Darkness, so did these warriors come hither with Crystals of Light.

Good assumption that all the Ascians have a Crystal of Darkness so they can hop across worlds, even if we only ever see very few of them.