The natural warmth and beauty of slate does not come with a high impact on the world's ecology.
Typical slate quarrying production does not involve any chemical processes. Generally only locally sourced water is used, to keep cutting machinery cool.
During excavation, unusable rock is excavated. The regional authorities have strict guidelines about how this material is treated. Often slate waste is returned adjacent to the quarry and the original landscape re-created with landscaping and the reinstatement of soil, grasses and other native vegetation.
In terms of sustainability, slate wins over other forms of roofing. Recent independent research has concluded that when compared to concrete or clay tiles, or to copper or lead, slate's minimal processing gives it the lowest embodied energy of all roofing materials. In addition, the same research states that its long life expectancy, low maintenance and recyclability give slate credibility in terms of environmental impact.
Due to the paucity of domestic slate supplies, specifiers are looking abroad for suitable alternatives to UK quarried slate. Spain is the UK's nearest meaningful source of slate. Slate from China or South America might have to be transported more than ten times the distance of that originating from Spain. So when CO2 emissions are taken into account, Spain is a clear winner.