No, they're not. That doesn't mean that cheap plastics are used;
in fact, the non-glass elements are usually considerably more
expensive (and more difficult to produce) than the optical glass
elements. Optical glasses are made in a number of different
formulations (such as crown glass and flint glass) that have
different optical properties, including different indices of
refraction (which has the effect of bending light more or less
with the same shape of lens) and different dispersion
characteristics (the amount that the light spectrum is spread
out). No single lens is perfect, so multiple elements of
different shapes and optical characteristics are used to correct
one another. The corrective elements are often made of exotic
non-glass crystals, like fluorite. Less often, aspherical molded
elements are cast from an optical resin (plastic, if you prefer)
bonded onto a more conventional glass element. (These days, the
cast aspherical elements are more likely to be glass, not so
much because resin is a bad thin