smyrlingsþumlar (mummy-thumbs, date-fruit)

The word date originates from the Latin dactylifera, which means "finger-bearing".  But dates don't actually look like fingers, they look like a special kind of fingers, they are "thumb-shaped'.  And they look brown and wrinkly, like the thumbs of a mummy.  The term "mummy-thumbs" actually pack a remarkable lot of information about how dates look like: The first element smyr(ð)lingur, "mummy" refers to Egypt, which is situated in that part of the world where dates are cultivated, while it describes the brown colour and wrinkly surface of the fruit, while þumlar, "thumbs" is descriptively more precise a reference to the etymology of the word "date".

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)

þolkíslungur (topaz)

There is no Icelandic term for "silicate" or "silicate mineral", although it is relatively easy to construct one.  The Orðaskrá um eðlisfræði og skyldar greinar mentions kísl for silica (SiO2, silicon dioxide).  Silicates are compounds with silica or oxidized silicon, so all we have to do is adding -ungur or
-ingur to kísl: kíslungur.  compounds: e.g. neosilicate = nýkíslungur.  The topaz is the hardest silicate mineral in nature.  Initially I thought of kóngakíslungur, harðkíslungur, but the best choice in my opinion is þolkíslungur.