Lagh metal is a fictional extraterrestrial substance featured in the story "Out of the Aeons", written by H. P. Lovecraft for Hazel Heald.
Description
Lagh metal doesn't occur naturally on Earth, but has been brought to the ancient continent of Mu by alien creatures from Yuggoth. The cylinder that contained the scroll of T'yog was made out of this substance, which defied all kinds of scientific analysis. The metal is further described as iridescent and "impervious to all reagents". (HPL: "Out of the Aeons")
Behind the Mythos
In The Whisperer in Darkness, the Mi-Go are known to use brain cylinders made of an unknown metal from Yuggoth. Ramsey Campbell's "The Mine on Yuggoth" names this metal tok'l.
The Tulu-metal used by the K'n-yanians from The Mound is also unknown to Earth, having been brought from the stars by Cthulhu, and is similarly used to make cylinders to store important scrolls.