The properties that you see by eye are only a small fraction of all the
properties of a material. Although diamond and glass both look about
the same (almost transparent, somehwat refractive) that doesn't mean
that they have much else in common.
Diamond consists of a periodic 3-D arrangement of carbon molecules,
connected by covalent bonds. Typical glass consists of amorphous
silicon dioxide, usually with various other compounds such as sodium
hydroxide. By amorphous, we mean that the atoms do not form a repeating
pattern but rather are somewhat irregularly arranged. So there's no
particular reason for any of the mechanical properties of these two
completely different materials to be similar.
Why are they both transparent? Both are uniform enough on the scale
of the wavelength of light not to scatter the light of small
impurities. Actually, the amorphous structure of glass is less uniform
and does scatter a little, but not enough to easily notice by eye.
Diamond has only one type of atom and only one type of bond. That
leaves a relatively small number of different excitation modes. It
turns out that it just doesn't have any vibration mode that absorbs
energy in the frequency range of visible light.
Standard glass has a bigger range of modes, but usually nothing
much that absorbs a lot of visible light. Thick glass can look slightly
greenish however, and of course some glasses are strongly colored.
Diamond has a higher index of refraction than ordinary glass, and
is also more "dispersive" -- the index of refraction of diamond varies
more with the wavelength of the light than does that of glass. This
makes diamonds spread the colors of light through larger angles if it
is cut in a shape of a prism, or in fancy, expensively-cut diamonds,
into a large array of angled surfaces. The high index of refraction
also increases the amount of light which bounces internally off of a
cut surface -- the buzzword here is "total internal reflection". These
properties make cut diamonds look brighter and more colorful than a
similarly cut piece of glass.
(republished on 07/17/06)