Among natural phenomena (says Sir David Brewster) illustrative of the
colours of thin plates, we find none more remarkable than one exhibited
by the fracture of a large crystal of quartz of a smoky colour, and
about two and a quarter inches in diameter. The surface of fracture,
in place of being a face or cleavage, or irregularly conchoidal, as we
have sometimes seen it, was filamentous, like a surface of velvet, and
consisted of short fibres, so small as to be incapable of reflecting
light. Their size could not have been greater than the third of the
millionth part of an inch, or one-fourth of the thinnest part of the
soap-bubble when it exhibits the black spot where it bursts.