A Jelly-fish

Visible, Invisible,
a fluctuating charm
an amber-tinctured amethyst
inhabits it, your arm
approaches and it opens 
and it closes; you had meant
to catch it and it quivers 
you abandon your intent.
Marianne Moore, 1887-1972. For more info, please see poets.org.
This poem has been rattling around my head for days.