This is Crotal, a lichen which is commonly found – throughout the Outer Hebrides – on rocks, gravestones and even on roofs – and which was traditionally used for a variety of colours, depending on the process. (It also depends on the exact species of lichen, as there are many that look extremely similar. J thinks this one may be Xanthoria pariontina – Maritime Sunburst lichen – but then says there’s many other likely candidates!) One traditional recipe involves fermenting an orange-brown lichen in urine, to produce a purple dye!
To give you an idea of scale, the small ‘cups’ at upper left are less than half a centimetre across. These lichens take years and years to grow, and so we never take but a small proportion of the lichen found at any one place.