Magi Gems

Magi Gems
Magi gems are stones used by mages to create magic and cast spells.

Background
The ore that is used to make magi gems is said to be given by the goddess, and are found in lands typically inhabited by elves.

Magi gems start out clear, but when someone is bonded to it it develops an aura. Auras come is a variety of colors and aren't known to change. Auras have a variety of colors.

Once a gem is bonded to someone, no one else can use it.

When in use, a magi gem will produce a magic circle or siguel. It is the same color as the gem and it's aura and has a pattern in the middle. Most humans do not use siguels.

There is some skill involved in using magi gems, as it is implied that some mages are more powerful than others. It is also stated that no one (with an exception of Vector and Marina) is able to use magic without one.

It is implied that you have to take a test in order to be able to get a magi gem. Most civilians don't pass the test. In addition to that, magi gems are hugely expensive. Between the skill required to pass the test and the price, it appears that the majority of mages work for the capital.

Arianna
Arianna has a bright red magi gem that she uses frequently. It is quite large compared to others that are meant for civilians. As Arianna makes most of her own spells, she has a siguel. Her siguel's pattern is also much more complicated than others.

Zera
Zera has a green magi gem. He is apparently very skilled with it.

General Brisse
Brisse has a reddish-purple magi gem. He is apparently very capable of using it, though there are very few instances where he is shown using it.

Sonja
Sonja has a purple magi gem that she is very skilled with, and uses it to create arrows and other weapons.

Terran
Terran has a brown magi gem. He is not seen using it; while he is a mage, he specializes in hand-to-hand combat.

Other
As mages aren't uncommon, there are other characters that have magi gems:
 * All known mages (aside from Arianna) are affiliated with the capital.

Trivia

 * People that work for the capital have pointed gems, while civilians have round gems.
 * Magi gems are seen only as necklaces in Castoff, but there is a possibility that there are other ways to wear them.