The Glossary of Rocks and Minerals says that
invisible structures in crystal
explain the qualities:
gravity and hardness,
the fatal habits, why
invincible diamond
will cleave along axes
of symmetry too small
to be seen.
                    But threshold,
where the eye unaided
apprehends transparence,
brilliance, the glide across
visible surface, its
lexicon of flatness,
this world, informs the intention of
the crystallographer, too far out, on the ice.