Flowing Lavas
We do not know if any of the gabbro magma was erupted from the Rum volcano; if it was erupted, the resulting basalt lavas have long since been eroded away. However, lavas that were erupted from later volcanoes - possibly those on Skye and Mull - can be seen in the west of Rum, where they form the top parts of the hills of Fionchra, Orval and Bloodstone Hill. On the west side of Fionchra, the lavas have cooled into a columnar structure like that at the Giant's Causeway in Northern Ireland. These lavas lie on top of sedimentary rocks called conglomerates, which contain many rounded pebbles, laid down in rivers flowing across the land surface before the lavas were formed.
The pebbles in the conglomerates actually include pieces of Rum gabbro, and this tells us that the conglomerates, and thus the lavas on top of them, are younger than the igneous rocks of the Rum Cuillin. When the lavas were erupted, the flowed down the ancient river valleys, and the shapes of these valleys can still be seen today, particularly on the slopes of Bloodstone Hill.
At Bloodstone Hill, holes and cracks in the rock are filled with green agate which contains many tiny red flecks, and this is thus known as bloodstone. This agate is composed mostly of silica, and was formed from heated water flowing through the rocks. This water dissolved minerals from the surrounding rocks, then precipitated them again into cracks when it cooled. The red flecks are tiny crystals of iron minerals, which have been oxidised on exposure to air, turning red.