Cinnabar is a mineral of a strong signature red hue that produces the pigment called vermilion. The word cinnabar allegedly comes from the Persian word for "dragon's blood."
I have already mentioned one of the Four Symbols in Chinese mythology, the Azure Dragon of the East. Well, the Vermilion Bird of the South is also one of them, and is fabled to live in a cinnabar cave somewhere in the South. Red is symbolic to this bird partly because its association with heat and fire, but I have always wondered if it has something to do with all the glorious tropical birds of red hues of South-East Asia. But I digress.
What does cinnabar have to do with immortality?
According to the legends of the Eight Immortals, the oldest person to achieve immortality was Zhongli Quan (usually translated as Iron-Crutch Li). He lived during the Han Dynasty, and being a miracle child advanced quickly in the royal court until he became a general of the armies. After being beaten in battle by the Tibetan army, he hid in the mountains and became the apprentice of a mysterious old man who taught him the secrets of alchemy (in a three-day intensive boot camp): how to turn stone into gold and silver, and how to prepare the Elixir of Longevity from cinnabar. Equipped with a fan that can turn stone into gold and silver, he set out to make the world a better place. In time, Iron-Crutch Li achieved immortality and ascended into the Heavens on a shining cloud, becoming the leader of the group of Eight Immortals.
WARNING: Do NOT try this at home. Cinnabar contains mercury and is highly poisonous. Any attempt at immortality might be very damaging to your health.
Fun fact: The Barony of Cynnabar, one of the baronies of the Middle Kingdom within the Society of Creative Anachronism, choose its name based on the symbolism of cinnabar, and especially because the symbol of the kingdom is a red dragon. Read the full story here.
Also, talking about red hues that start with C, check out Andrew Lang's classic Crimson Fairy Book.