From Guild of Archivists

Nara (nara[contested]) is the hardest of all D'ni stones. This synthetic stone is described as being dark gray-green or metallic gray in color, and possesses a density 30 times that of steel[1]. It was created in 5307 DE by the Guilds of Miners and Stone-Masons in a joint development project. A fusion-compounder is used to manufacture it by reforging the atomic links of regular stone.

The D'ni considered nara to be essentially indestructible. They used it for secure locks, shoring up and sealing tunnels, and other tasks which required materials of incredible durability and strength. It is not without its flaws, however. Sirrus, in a bid to escape his prison on Spire, discovered a sonic frequency which would disrupt the material on a molecular level, causing it to shatter like glass. In this way, he was able to destroy the nara chamber protecting the link back to Tomahna and earn his escape.