Optolythic data rod
From Memory Alpha, the free Star Trek reference.
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 extraordinarily 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 with a light blue tint. (DS9: "In the Pale Moonlight")
