Grathon Tolar, holding an Optolythic data rod
An optolythic data rod is Cardassian technology that is used to store information and records. These data rods are unique in that they are only produced on an as-needed basis, and information can only be inscribed on them once and cannot be altered thereafter. As a result, these data rods have a reputation for being a secure way to store sensitive data. They are extrodinarily difficult to obtain, and nearly impossible to counterfeit.
As the name suggests, they are long, thin and cylindrical in shape and unlike standard isolinear rods which are a translucent dark amber, optolythic data rods are clear. (DS9: "In the Pale Moonlight")