pyrite (glópagull, brennisteinskís)

The term pyrite in its original meaning, before its meaning was extended to a large class of minerals, designated iron sulphide, which has a golden colour and was often mistaken for gold, hence the name fool's gold, of which the Icelandic glópagull is a literal translation.  But there are many more possibilities to form Icelandic names for this mineral:

hræsnistár freyju (the tears of the godess Freyja became gold, so her crocodile-tears must have turned to "fool's gold".)
hégrátssteinn (hé (false, as in hégómi, hégilja) + grátur)
vanynjublendi (= blende of the Waness).  In old mineralogical terminology, "blende" was originally used for Zinc sulhide, but also more generally to denote minerals that are deceiving, mostly metal sulphides, that didn't yield the right metal.  See: http://de.wikipedia.org/wiki/Blende_(Mineralogie)
Pyrite has all the properties mentioned it the German wikipedia article to be regarded as a blende: sulphide, metal-like, yellow or black, deceiving. 
brennigull (brimgold): Pyrite was named after fire, it is a compound with "brennisteinn" and was used as a raw material to produce sulphuric acid (brennisteinssýra)
Pyrite is the most common of the sulfide minerals. The name pyrite is derived from the Greek πυρίτης (puritēs), "of fire" or "in fire", in turn from πύρ (pur), "fire". In ancient Roman times, this name was applied to several types of stone that would create sparks when struck against steel.  (see http://en.wikipedia.org/wiki/Pyrite )

Surtarjárn: Iron of Surtr (constructed like Surtarbrandur)