Reachable only by boat or by a demanding three-hour trek from the Altopiano del Golgo, Cala Mariolu is arguably one of the most sublime spots on the coast. Split in two by a cluster of bright limestone rocks, it has virtually no sand. Don’t let the smooth white pebbles put you off, though. The water that laps these beaches ranges from transparent white at the water’s edge through countless shades of blue on to a deep purplish hue.