Poems have been written on whatever materials were at hand, and with whatever writing instruments could be found or fashioned. I'm sure if Shakespeare have been in prison, he would have found a way to write, if only to while away the time. When it comes to prison poets, long works have been written on toilet paper.
One writing material, and writing instrument, used by prisoners of some repute were diamonds on glass. Odd.
I can think of only two instances where a prisoner (both were female) wrote on the glass window of her "cell" with the diamond ring on her finger. Both were imprisoned royalty (hence why they still were allowed to wear their diamond rings, I guess.)
Most recently we have the Empress Alexandra of Russia. She scratched her initials in the glass of the window where she and her family were confined in Tobolsk in 1917 or 1918. Not very inventive, If I do say so.
More inventive, or at least more poetic, was the couplet inscribed on her glass window at Woodstock (the manor, not the music love-in in New York) by prisoner-princess Elizabeth, daughter of Henry VIII, in 1554. The soon-to-be Elizabeth I scratched these concise (if not contrite) words:
Much suspected by me,
Nothing proved can be,
Quoth Elizabeth prisoner.
Okay, not a couplet. Turns out she was right, though.
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Incredibly useful Renaissance trivia tidbit:
"[In childhood], Elizabeth was well-schooled in the arts. An excellent keyboard player, and a lover of dancing even in later life, she encouraged and carefully planned music for ceremonial occasions throughout her reign."
