One of the most common misconceptions about rocks is that they are all grey. Perhaps this is one of the reasons that, to many non-geologists, the thought of studying rocks for a living is utterly incomprehensible.
And whilst I personally think rocks are intrinsically fascinating for many things other than their appearance, this monochromatic version of geology simply isn’t true. Allow me take you on a kaleidoscopic tour of the visual beauty of rocks.
Rocks, by definition, are aggregates of minerals. There are nearly 4,000 minerals on Earth, spanning the whole colour spectrum.
Minerals are inorganic solids that have a defined chemical composition and an ordered chemical structure; these two properties are the key to the their beauty.
Different minerals absorb certain wavelengths of incident light, with the remaining part of the visible light spectrum being perceived by our brain as the colour of the mineral. This absorbtion can be influenced by many factors of the mineral’s chemistry or structure. For example, electrons orbiting transition metal atoms are particularly prone to being excited to a higher energy state by the absorbtion of electromagnetic radiation. This makes them particularly adept at colouring minerals. Trace amounts of transition metals give rise to an array of colours in quartz (e.g., purple amethyst: iron; yellow citrine: ferric iron; rose quartz: titanium, iron and manganese) and corundum (ruby: chromium; and sapphire: titanium and iron).
The structured ordering of atoms in many minerals is manifest in spectacular crystal form, and allows the colour to be seen more readily by the naked eye.
With such polychromatic ingredients, it is hard to imagine how rocks got their reputation for being so boring! Perhaps the most obviously colourful rocks are metamorphic, their original constituent minerals being replaced under the duress of heat and pressure by more exotic and vivid counterparts.
Out of the igneous group, plutonic rocks (those that formed through cooling of magma below the Earth’s crust) are the obvious flag-bearers for aesthetic beauty. Their coarsely crystalline textures are the perfect vehicles to show off the many hues of the rock-forming minerals. But volcanic rocks are not to be left out. Whilst most lavas do appear at first sight to be a shade of grey, a closer look will commonly reveal phenocrysts or voids filled with secondary mineral deposits (amygdales).
Even sedimentary rocks are prone to flashes of colour. Consider the dazzling white chalk cliffs of Dover, the bright orange New Red Sandstone, the scarlet of Banded Iron Formations, the bluey-black of oily shales…
So you see, with geology you really can taste the rainbow.