Fire Crystal is the heart of a Phoenix and source of the fire which keeps them alive. The crystal is also what is left from a Phoenix when the fire burns out and from what they can be reborn after coming in contact with fire. Skilled mages can block the transformation turning the crystal into an almost infinite energy source.
You are using a fire crystal!? This is a living, intelligent being!
History & Usage
It is unknown exactly when the way of blocking the crystal's transformation was disovered. People were always aware of the crystals, but didn't know how to use them. Currently, the secret of blocking the transformation has been known for a few hundred years. However, due to the complicated nature of the process and ethical objections by some people, the crystals never became a common power source.
The complex nature of acquiring a crystal and getting it work, makes it a very expensive product. Only the wealthiest people can afford it. Those people usually use them as a power source for a residence or a factory. In a factory, a crystal replaces a steam engine. The only problem which occurs then is that most factory equipment is made to work with steam engines and requires a lot of changes to get it to work with a fire crystal.
About 20 years ago, a tarranoi inventor built the first ever airship and used a fire crystal to power it and make it fly.