Silicon and Germanium

1) Carbon: kolefni
1) diamond: alsverfir (scratches all other substances because it is the hardest stone), kóngasteinn (king of stones, most noble gem), þolefni (pun on kolefni)
2) graphite: ritkol, ritblý
3) graphene: netkol

2) Silicon: synonyms for kísill could be: sandkol, sendi (the carbon in "sand" (= SiO2) jarðmálmungur (the earth-metalloid, most abundant metalloid in the earths crust), frummálmungur (the first metalloid to be formed in the universe, boron was formed afterwards in a process called "cosmic ray spallation".

3) Germanium
1) blendiskísill:
In the same way as the word "salt" in it's original and narrowest meaning designates NaCl, the German mineralogical term "blende" denotes "sphalerite" or "zinkblende" (zinc sulphide, ZnS), the most well-known of the "blendes". The English name of the mineral "sphalerite" derives from Greek "sphaleros", treacherous, and is semantically equivalent with the German "blende", which was coined by Agricola in 1546, and is from the derived from "blenden" (to blind, or deceive), because the mineral resembles lead-ore in appearance but contains no lead, and was consequently oftep rejected as worthless . Today, germanium is mined primarily from sphalerite (the primary ore of zinc), though germanium is also recovered commercially from silver, lead, and copper ores.  See http://en.wikipedia.org/wiki/Germanium
For that reason, one could call germanium "blendiskísill" (the "silicon" from sphalerite)
2) þýskakísli: This is a funny sounding word, which we call "tickle-words" (kietelwoorden) in Dutch, like "kanunnikenkapittel", which translates as "kanúkakapitúli" and I have a feeling this word rolls from the native Icelandic tongue as sweetly and smoothly as in the case of the originally Dutch construction. Maybe the average sense of humor of the "market", as Icelandic terminologists call the general public with regard to the acceptance of newly-coined words, will help to get this one through, who knows?
3) hálftin: The prefix ‘hálf’ refers on the one hand to the semimetallic properties of germanium and on the other to its being situated exactly in the middle between silicon and tin. In this latter sense the prefix is used in the same way as hálf- in hálfþrítugur. The prefix perfectly denotes its intermediate position, being chemically homologous with silicon and tin.