The Blazing Ruby is a jewel that was found on The Fallen Star, and is used by Midnight Yasai to amplify her magic and Matrixz Control. She uses it in a pendant and it's necessary an incredible power to destroy the jewel. For some reason, the Blazing Ruby can't amplify any other magical energy other than Midna's, but it's necessary for her to be wearing it to be effective, or else the Blazing Ruby is just another common Jewel in a Silver Pendant. The most probable reason for that, is the fact the Jewel was extracted from The Fallen Star, the place where Midna was found as baby.
How it works
The Blazing Ruby spontaneously amplify the energy inside Midna, making her magic and Matrixz Control more stronger. It can also absorb energy from spells of another Sorcerers and use the same spell against them, in a counter move, although being very hard to succeed in a maneuver like that.
The more stronger is the spell, then more will need the Blazing Ruby energy, and in cases like that, the jewel starts glowing, hinting to be giving its all to make the spell work.
Another moment when the jewel might glow, is reacting with a pulsing light, when there's an imminent danger for the wearer.