An optolythic data rod was Cardassian technology used to store information and records. These data rods were unique in that they were only produced on an as-needed basis, and information could only be inscribed on them once and could not be altered thereafter. As a result, these data rods had a reputation for being a secure way to store sensitive data. They were extraordinarily difficult to obtain, and nearly impossible to counterfeit.
As the name suggests, they were long, thin and cylindrical in shape and unlike standard isolinear rods which were a translucent dark amber, optolythic data rods were clear with a light blue tint.
In 2374, Garak, acting under instruction from Captain Benjamin Sisko managed to obtain an optolythic data rod as part of an elaborate plot to bring the Romulans into the Dominion War. The source demanded 85 liters of bio-mimetic gel, which Sisko wanted to refuse but was persuaded not to since the rod was necessary for the plan's success and obtaining it was "a minor miracle." Unfortunately for them, Romulan Senator Vreenak discovered the data contained on the rod to be a fake; however he was killed before he could report the information back to his government. (DS9: "In the Pale Moonlight")