Tallonian crystals are a type of gemstone found on the Tallonian homeworld. It is illegal to transport Tallonian crystals off their native planet.
Quark attempted to purchase Tallonian crystals in 2373, but was stopped from doing so by Odo. Worf had been alerted to Quark's intention after he learned Quark was trying to obtain a Tallonian microscanner. (DS9 episode: "Hippocratic Oath")
In an alternate timeline, Nancy Hedford negotiated with the black marketeers on Epsilon Canaris III, striking a bargain that made numerous alien goods, such as Tallonian crystals, legal to trade in United Earth territory. (Star Trek: Myriad Universes novel: A Less Perfect Union)