Darksteel itself was mentioned in Volo's Guide to All Things Magical along with many other wondrous materiels.
Darksteel: The composition of this alloy is (or was) a secret of
the possibly extinct Ironstar dwarf clan. It is unheard-of to find a
new item made of darksteel, though a rare and lucky few delvers
into deep places have come upon ingots of darksteel. (A darksteel
ingot is about as large as the empty area in the middle of four rectangular
human warriors’ shields stood up long-edges-together to
form a hollow square when viewed from above.)
Darksteel behaves like steel but is lighter, and when coated in
certain oils whose formula is known to few52 and heated in even a
small fire, it becomes molten, and can be poured into molds—
even simple sand molds—to be cast into a new shape. Remarkably,
this does not destroy or (usually) alter existing enchantments
on the metal. Moreover, darksteel is durable and takes new
enchantments readily, even if it is already dweomer-laden. All of
this makes darksteel very valuable (about 10 to 12 times more than
gold, by weight), and it is eagerly sought by dwarves and by adventurers
of other races. Certain smiths in Neverwinter and Waterdeep
have standing offers of 7,000 gp or more for an intact
darksteel ingot.
Darksteel is silvery in hue when polished or cut, but its
exposed surfaces have a deep, gleaming purple luster. It has the
unique property of absorbing all natural and magical lightning or
electrical energy (not heat, and not magic missiles or other pure
energy pulses) into itself, without conducting any of it—or its
damage—to a wearer or being in contact with it.
Items made primarily of darksteel automatically succeed in all
item saving throws vs. fall, normal fire, cold, and electricity. They
receive a +3 bonus to all item saving throws vs. acid, crushing
blow, disintegration, and magical fire, and a +5 bonus to all item
saving throws vs. lightning.