Duranium is an extremely strong metallic substance that was used in alloys for hull construction of the starships and space stations of numerous cultures, including Andorians, Cardassians, Ferengi, Humans, Klingons, and Trabe.
Duranium interests were traded on the open market of the Ferengi Futures Exchange. Over the last three trading cycles prior to mid-2373, there had been an increase of 7.75, a decrease of 2.125 and another increase of 11.375, making an overall gain of seventeen points. That year, DaiMon Igel petitioned to Grand Nagus Zek as to whether he should sell his duranium interests on the Exchange. (DS9: "Ferengi Love Songs")
Portions of the interior of Galaxy-class starships were composed of solid duranium. It was discovered, in 2366, that Krieger waves were capable of putting a hole in solid duranium, something that was rather difficult to accomplish, when one was discovered in the cargo bay of the USS Enterprise-D. (TNG: "A Matter of Perspective")
Portions of the secondary hull on a Klingon Vor'cha-class attack cruiser were partially constructed from duranium alloy. While searching the debris of the IKS Toh'Kaht for a mission recorder, Dax noted that the magnetic field from the duranium might be masking it. (DS9: "Dramatis Personae")
Duranium alloy was not normally used in the construction of the pylon supports of class 2 shuttles, making the shuttle susceptible to subspace torque when trying to achieve warp 10. Harry Kim suggested using a duranium alloy to reinforce the nacelle, but B'Elanna Torres had already considered that and determined that it was "too brittle." (VOY: "Threshold")
The Cardassians used two-meter thick duranium composite to shield all the access conduits aboard Terok Nor. Prior to the station's encounter with the Hunters, Miles O'Brien had "never seen a scanning device that could penetrate it." (DS9: "Captive Pursuit")
Deep Space 9's crew later took advantage of knowing that this composite was impervious to the station's own security net scans and hid in the conduits in 2370, when the station was under siege. (DS9: "The Siege")
The emergency hatches aboard Federation runabouts were made of duranium composite. When Major Kira Nerys suggested to burn through the hatch, Commander Benjamin Sisko told her to "forget it" and that "it would take [her] an hour to get through." (DS9: "Q-Less")
Duranium shadows can be used to create the illusion, to sensor scans, of large weapon stockpiles. In 2369, Miles O'Brien created such an illusion behind a thoron field to fool Gul Jasad into thinking that Deep Space 9 was far more heavily armed than originally believed. O'Brien's duranium shadow gave the Cardassians the impression that the station was armed with an estimated 5,000 photon torpedoes and integrated phaser banks on all levels. (DS9: "Emissary")
Prior to the first Battle of Deep Space 9, Benjamin Sisko warned Gowron and Martok that the station was armed with 5,000 photon torpedoes, and suggested that if they did not believe him that they were free to scan the station. After the scan, Martok was still skeptical and stated to Gowron that it was a trick, and that what they were reading was likely an illusion created by thoron fields and duranium shadows. (DS9: "The Way of the Warrior")
According to the Star Trek: Deep Space Nine Technical Manual, duranium is a naturally-occurring ore in planetary crusts such as the crust of Bajor. This would suggest that duranium itself is not an alloy, but rather that there is a family of elements where duranium is the prime constituent. This is further supported by the use of the term "duranium-235," which implies that duranium is in fact an element, one that has an isotope with an atomic weight of 235.
This material is also referenced as a construction material in the video game Unreal Tournament 2004 and in Star Trek: New Worlds as a sixth resource material refined from dolamide ore. It is used to construct most structures.