Crystals

Crystals are used for their ability to harness natural forces. They grow in the inside of the volcano and due to their proximity to the Radiant Phoenix,s nest are imbued with power from that.

Use

Crystals come in all colours of the rainbow. Each family prefers those in their colour, so they can match their clothes. They are set into jewellery in such a way that the crystal is in contact with the skin, to allow the power stored in there to be accessed.
Generally the bigger the crystal, the more power can be channelled through it. But different users are able to get different amounts of power from the same crystal.
Headbands with at least one central stone are the most common, as they double up as a way to secure head scarves needed in the desert winds. But ear cuffs, ear rings, rings, and necklaces are also ornamented with crystals. Necklaces are the least common, as they can get caught in your clothes, or move away from the skin, losing contact and stopping the wearer from drawing on their power. Crystals are also added to other items held in the hand, like staffs, or daggers.

Harvesting

The best crystals are harvested from the walls of the caves, like the Shautane Grotto, deep in the volcano. These have spent the longest near the Phoenix and have the most power to draw from. But visiting there is treacherous, as the mountains are full of wild creatures, as well as the phoenix itself which vigorously defends it's territory.
Over time, the crystals fall from the walls, washing out of the mountains with the rains. They can be found along the River Beach, or panned out of the sediment at the bottom of the water.
The emperor insisted on an equal allocation of crystals found after the rainy season between the different families, but that rule has fallen out of favour since the The Final Emperor passed.

Use

The crystals give you the ability to move things, or hold them, that are beyond your reach. To solidify air into a shield, to pull things towards you. To freeze a person, to split the raindrops so they don't fall on you, to change the colour of things, to stop things falling, to collect things together, etc
If you draw too much power through it, then you will feel fatigues. Where that line is for each individual is different, and one of the key things when learning to use it is to find that line for yourself.
The larger, and clearer, the crystal, and the stronger your connection to it is, the more power you can draw through it. Connection is genetic, and is based on how close your ancestors sat to Horaeri, The First Emperor when the power was transfered to him by the Radiant Phoenix.