The word in the title is pron. ˈmʌndɪk and is thought to derive from a Cornish compound noun, the components of which are men aka maen aka mên, meaning “stone” and teg, meaning “beautiful”. The term was applied to various forms of pyrite found in tin and copper mines. Iron pyrite is commonly known as Fool’s Gold. You can see an example of a pyrite in the picture. Pretty it may be, but its name is now infamous in these parts.
Around 1900 some bright spark had the idea of making concrete out of mine waste and then building houses with it. The waste matter often contained mundic. The problem is that pyrite reacts with water and oxygen causing expansion of the material and eventually your house falls down. The practice was halted in 1950 and all houses in this region built with concrete before that date need to have a mundic survey carried out before mortgage lenders will agree to cough up funds for the property.
Thanks to JDL for reminding me of mundic.
Photo credit: Rob Lavinsky. Used under this licence.