Granite, Mourne Mountains, County Down

The granite that forms the Mourne mountains was originally intruded deep into the Earth’s crust at around the same time (around 56 million years ago) that the volcanoes which formed the Giant’s Causeway were erupting further to the north.

Normally the crystals that make up granite - mostly quartz and feldspar with some mica - interlock tightly together, which is why granite is such a durable rock. In some places, however, bubbles of gas in the magma left pockets that became lined with beautiful crystals, such as smoky quartz, green beryl and colourless topaz - mistaken in the past for diamonds!