Helgustadir is the name of a farmsite on the Northern shore of the Reydarfjordur in the extreme East of Iceland. This deposit of outstanding transparent calcite was discovered in the 17th century and was intermittently mined for many decades until 1924. The transparency of the crystals were ideally suited for the optical industry and therefore the calcite quickly became famous as "Iceland spar" and its marvelous crystals were rapidly distributed among the leading European scientists of that time. The initial concepts of double refraction and crystal cleavage planes were first discovered on calcite crystals from this site and led to a far better understanding of crystal physics. Hence Helgustadir is one of those classic sites, which started mineralogy as a serious science.

The calcite crystals, which are accompanied by small tufts of stilbite crystals reach a size of several meters. RÖSLER (1981 : Lehrbuch der Mineralogie) quotes a maximum size of 6 x 2 m , but dont give any more details. Other authors describe 7 x 7 x 2 m large crystals. A crystal of 230 kg is said to be on display in the exhibition of the Museum of Natural History in London.

Today there remain large dumps of the former mining activity, whereas the mine as such is most of the year filled with snow and more or less inacessible. It is still possible to find small scattered cleavage pieces of double spar calcite on the dumps, especially after periods of heavy rain, which is not a rarity in Iceland. However collecting in this mine is forbidden by law, as Helgustadir was declared a national heritage site in 1975.