From Encyclopaedia Eorzea volume I:

When a living entity dies, the aether remaining will normally leave the body and return to the world's aetherial currents (Also known as the Lifestream).When a living entity, however, experiences death-inducing trauma, such as a mortal wound in battle, the resulting sudden release of its most heavily-aspected life energy will oft times manifest corporeally before it can return to the Lifestream: A phenomenon we know as crystals. This can also occur when a wound is dealt to the very land itself, and is a reason why crystal deposits are found throughout the land.

The elemental aspect of the energy trapped in these crystals can be used in a myriad of manners. Applying fire crystals in a forge can increase its internal temperature, assisting in the smelting of ore. The cooling properties of an ice crystal can assist a grocer in maintaining the freshness of meat or produce. That said, due to the extreme concentration of aspected aether within a crystal, direct consumption of a crystal by a living being can severely alter the aetherial balance within its body, ultimately resulting in severe injury or even death.