Diamond is the hardest crystal.  It is also, according to a recent article in Scientific American, the most perfectly structured crystal, by which they mean it has a perfectly symmetrical crystalline matrix structure.  It reflects light better than any other substance.  The name comes from the Greek word adamastos: "unrelenting, unconquerable", which originally meant a state of being to which every man should aspire.
    It's strength makes diamond a symbol of fearlessness and invincibility.  It was thought to give warriors strength and courage.  It was also believed by some to scare off ghosts and evil spirits.  St. Hildegard said the devil hated this stone because it resisted his power.
    In the Talmud, a hight priest wears a gem, believed to be a diamond, that reveals the guilt or innocence or someone accused of a crime.  In Hindu lore, a diamond bearer is protected from fire, poison, thieves, water, snakes and evil spirits.  Plato said diamonds were living being containing celestial spirits.
    According to another recent article in Scientific American, in a few years we will be using computers with memories that are small, man-made diamonds.

Diamonds come in many colors, but the clear or blue/white is most common.  As a clear or white crystal, it primarily stimulates the crown chakra at the top of the head and would most benifit physical illnesses in the head.