Quartz can be crystalline, or amorphous. The amorphous form, after softening
in a flame, can be pulled, like taffy, to form filaments. But, it's
rather more exciting than that; in the words of C. V. Boys Sci. Am. Supplement 717
The apparatus consists of a small crossbow, and an arrow made of straw with
a needle point. To the tail of the arrow is attached a fine rod of quartz
which has been melted and drawn out in the oxyhydrogen jet. I have a piece
of the same material in my hand, and now after melting their ends and
joining them together, an operation which produces a beautiful and
dazzling light, all I have to do is to liberate the string of the bow
by pulling the trigger...
In this way threads can be produced of great length, of almost any
degree of fineness, of extraordinary uniformity, and of enormous strength.
The resulting quartz fiber is tempered by its rapid cooling in air, so that
the surface of the fiber is held in compression. Because the surface chills
first, it freezes at a large diameter, and the material inside the
surface, as it shrinks and cools, pulls radially on that surface.
This means the resulting fiber, like tempered glass, is nearly immune
to surface cracks, or even microcracks. Cracks only grow when the material
is in tension, never in compression.
Quartz fibers made by this process are devoid of imperfections (because
any nonuniformity of the material would cause a break during stretching),
and nearly immune to microcracks, and have no ductility to speak of.
Crystalline materials can fracture on crystal planes, metals can exhibit
ductile flow, and most materials have microcracks which can pick up
contaminants and create weaknesses.
According to the inventor of this kind of fiber, it's the best available
torsion spring (and his other accomplishments seem to prove that point).
Best, though, doesn't mean 'most elastic' in the sense of great
elongation possibilities; it just means good conformance with
ideal spring behavior.