The island of Tenerife in the Canaries was formed by a volcano millions of years ago and evidence of the eruption can still be seen all along the coast as here at Costa Adeje where the different layers of sediment are clearly visible. The beaches are black sand and these rocks have at least 3 different colours.
I picked up a piece of rock from the base of the cliff to take back to draw and found it to be full of holes and tiny stones and pebbles. It's very crumbly which surprised me as this stone is used for buildings all over the island. Maybe there are harder versions that are specially quarried for this purpose.