A Cardassian data storage rod, almost impossible to forge or obtain, that Garak procures so that a faked recording of a Dominion meeting can be passed off to Senator Vreenak as genuine.
An optolythic data rod is a Cardassian recording medium used for official records. Each rod can be written to only once and is manufactured under tight state control, which makes a genuine blank one extremely hard to come by outside Cardassian space.
In the Deep Space Nine episode In the Pale Moonlight, Garak tells Sisko that a convincing forgery of a secret Dominion strategy meeting must be recorded on a real rod. The price demanded by his supplier is a large quantity of bio-mimetic gel, a controlled substance that Sisko obtains over Bashir's objections.
The forger Grathon Tolar records the holographic fake onto the rod. Vreenak examines it, declares it a fake, and dies shortly afterwards when his shuttle explodes, leaving the damaged rod as apparent proof of Dominion treachery.