Deretheni (DereTenE) is an artificial stone manufactured by the D'ni. It is described in The Book of D'ni as being jet black, and DRC research indicates that it was also incredibly lightweight, despite being nearly as strong as nara. It was discovered in 5236 DE by the Guild of Stone Masons, during the reign of King Emen. The Guild of Maintainers quickly noted that the stone would make an excellent material from which to create safer, more robust maintainer suits. These suits enabled Maintainers to inspect a wider range of Ages, thus granting the Guild of Writers more leeway in their Writing to include Ages with more experimental atmospheres and energy sources. These suits were used all the way up to the fall of D'ni, and even beyond, as Atrus and his restoration team used them to investigate Ages for possible survivors.
- "The jet-black overlapping plates of which [the Maintainer suit] was made had a polished, metallic look, yet there was no element of metal in their manufacture. The suit was made of stone—of a special lightweight stone named deretheni, not as hard as the legendary nara". Myst Reader. Ebook, The Book of D'ni "Part Three", section 3.