The Jewel of Charta was a small diamond-shaped gem from the Swamp Moons of Drenthax IV. It had the power to greatly increase and further strengthen a Tamaranean's natural abilities to great power-levels. To signify its power/ability-enhancement, her blackbolts turned red rather than light purple.
Blackfire was given the Jewel of Charta by Glgrdsklechhh, a slime/elephant-like alien creature, in a secret deal which involved Glgrdsklechhh getting married to Starfire. Blackfire tricked Starfire into marrying the alien creature, so that she could claim the Jewel of Charta from Glgrdsklechhh, so that she could rule Tamaran as a ruthless tyrant, and because no one would even dare challenge her with the mighty jewel in her possession.
When this was discovered by Starfire, the two sisters had a brutal fight with each other, which ended with Starfire defeating her elder sister and crushing the Jewel of Charta in the process, which reduced Blackfire's abilities to their normal level. However, while not wearing it anymore, her eyes still glowed red rather than their usual purple.