The Darkglass Mountains are a mountain range in the Feywilds which is made almost entirely out of a mysterious dark glassy substance. Unlike other mountains, which are made of several rocks and dirt and etc., the Darkglass Mountains seem to have been "carved" out of a single gigantic mass. There are no pebbles, no dust, no avalanches, just smooth (and occasionally sharp) glass.
In her book, Wondrous Sights, Lu'ul Ugaasi likened the mountains to The Gods' Jewelry, a nickname which has stuck.