I have a birth mark on the bottom of my right foot. It is a crescent moon reddish in color... it faces the same way a C would. I'm just curious if it has a significant meaning to it because I've always felt a strong connection to the moon and things like crystals and all that cool type of stuff.. !
Informally, I'd say the crescent can definite be taken as a lunar symbol, reinforcing your connection to the moon.
Regarding the letter C in relation to quartz crystals, this is much shakier because "crystal" is an English word, and has different forms in other languages: kristall is German for crystal; in Catalan it's vidre; in Chinese it's 水晶 (Shuǐjīng). You can see the problem.
There is a nice semi-mythical parallel using letters, in the sense that Shakespeare can be considered a myth-maker. (Mythology defined here from a strictly narrative, as opposed to religious, perspective:)
Richard plots to have his brother Clarence, who stands before him in the line of succession, executed over a prophecy that "G of King Edward's heirs the murderer shall be". Richard leads the king to interpret this as referring to George of Clarence, without realizing it actually refers to Richard of Gloucester.
(PS--don't trust wiki entirely. Always best to read or see the play. George of Clarence isn't drowned in a any old wine, but a "butt of malmsey";)