Crystals

[treuhavik]

What I found interesting was the "Dark" crystal seen in the lv15(?) main scenario quest; would have been at Swiftperch if from Limsa. The PC sees it after defeating the Ascian, but, before he/she is able to pick it up, it dissipates.

This means that there are light and dark, astral and umbral "elements" as well. Was this given any significance later in the story?

[Malrend]

What I found interesting was the "Dark" crystal seen in the lv15(?) main scenario quest; would have been at Swiftperch if from Limsa. The PC sees it after defeating the Ascian, but, before he/she is able to pick it up, it dissipates.

This means that there are light and dark, astral and umbral "elements" as well. Was this given any significance later in the story?

As I recall, it is mentioned by either Minfilia or Urianger, that these dark crystals are how the Ascians can manifest themselves in a host body.
As for the Astral and Umbral elements, they are sort of like umbrellas, in that they each cover a number of the elements. Like the THM/BLM's Astral Fire and Umbral Ice, unfortunately I can't recall which of the other elements belongs under which umbrella.

[Anonymoose]

I can't recall which of the other elements belongs under which umbrella.

There's an elemental wheel that has six elements in a hexagon, one on top, one on bottom, two on each side. Clockwise from noon, Lightning, Fire, Earth, Ice, Water, Wind, and back to lightning. The three elements on top of the wheel are "astral," existing primarily "above" - lighting, fire, and wind. The three elements on the bottom, existing primarily "below" - ice, water, earth.

Each "polar" element on the wheel is also part of a triad that connects it to the "side" elements of the opposite polarity. These triads are the elemental conquest and submission - lightning is grounded by earth which is grounded by water which is boiled away by lightning; ice is melted by fire which is extinguished by the wind which is obstructed by ice.

Honestly, it's about time I make a new graphic for that. Perhaps I'll make it in the style of the one that's in-game. Can you make out all of the things I said represented in the glyph? Notice how the wheel can represent the world - reality itself - with the warrior of light standing in the middle, where all of the elements are in balance and none is the dominant aspect.

Also notice how there's a mark for the space between elements around the Warrior of Light, drawing the number 12 into the design, as well...

However, Astral and Umbral exist in all things. There is an Ice Crystal of Light even though ice is an Umbral Element. There an astral and umbral month for every element, even though those elements are themselves astral or umbral. One should be careful not to confuse The Umbral with The Darkness.

As far as I recall at the moment, we've yet to see a crystal of Darkness that has an element, astral or umbral.

what exactly is their function supposed to be and why does your character collect them?

Most Useful Answer
Sadly, I don't have any more information than what I stated in my last post. Perhaps it's meant to keep a little of that magical mystery, for now.

Most Truthful Answer
Because XIV is a love letter to classical Final Fantasy and you will collect Crystals of Light, godsdamnit.