Westmorland Green Slate Also known as Honnister Slate, this is a variety of slate from the Borrowdale Volcanic Group of Cumbria. It has a distinctive green colouration due to the large amounts of the mineral chlorite contained within it. Unusually, it is formed from a volcanic ash deposit known as tuff. This volcanic sequence erupted during the Ordovician period of geological time (458 to 448 million years ago). The tuff has since been subjected to high pressure and heat, metamorphosing it to a slate. The resulting material has a unique texture making it a decorative gem material. The green-grey slates are from the lower seam quarried near the Honister Pass at The Honnister Mine. The finer grained green slates are from the upper seam near Ambleside. 

All search results