Gemstone prospectivity in Scotland

Scotland has never been considered a significant source of quality gemstones; nevertheless it is capable of producing a wide range of gem materials including agate, amethyst, cairngorm and garnet. Occurrences of more valuable stones such as topaz, beryl, sapphire, and tourmaline are far rarer, but the first two species along with cairngorm formed the basis of a thriving 18th and 19th century cottage industry in the eastern Cairngorms. The discovery in 1984 of 1-8 cm long gem-quality sapphire megacrysts on the Isle of Lewis was the first step in the realisation that the Scottish Highlands had commercial gemstone potential, particularly since one crystal generated a 9.6 carat cut stone valued at £60,000 in 1995. The megacrysts are hosted by a lamprophyre dyke that intriguingly has recorded a Tertiary age despite having much closer geochemical and structural affinities with the Late Palaeozoic alkaline lamprophyre suite. Confirmation of the age of the dyke is an important prerequisite in the identification of further exploration target areas. A single specimen discovered in Sutherland in the late 19th century was formerly thought to contain the country’s only example of diamond, but this has now been positively identified as colourless spessartine garnet. Recent advances in our understanding of diamond formation and the early geological history of NW Scotland suggest the required conditions might well have existed. In particular, geophysical evidence for a relict subduction zone, existence of thick Archaean crust and the recent discovery of a kimberlite-like dyke considerably enhances the potential of the district for diamond discoveries