Helgustadir is a farm on the northern shores of the Reydarfiord Bay. During a period of about three centuries, the best Iceland spar mine was exploited in the mountains above it. The mining started in the 17^th century and continued into the 20^th. The spar from the mine was and is mainly found in crevices, whereas it usually appears elsewhere as fillings in the pores of the rock. The largest crystals discovered in this country have been found there or in the Hoffell Valley in the Southeast. Most of the crystals displayed in museums came from the Helgustadir-mine. One of them weighs 230 kilograms and is on display in The Museum of Natural History in London.

Iceland spar was used for all kinds of precision instruments, such as microscopes, until synthetic materials took over. The crystal has a double reflection of the light, which gives the illusion that you see everything double if you lay a clear crystal on a written text. The Dutchman, Christian Huygens, researched the crystal very thoroughly and put forward the theory on the wave movement of the light at the end of the 17^th century in a very rare book called "Tractatus de Lumine".