This is a relatively small area of sand dunes comprising sand of seven distinct colors (approximately red, brown, violet, green, blue, purple, and yellow). The main feature of the place is that since these differently colored sands spontaneously settle in different layers, dunes acquire a surrealistic, striped coloring. Since the earth was first exposed, rains have carved beautiful patterns into the hillside, creating an effect of earthen meringue.
The sands formed from the decomposition of volcanic rock (basalt) gullies into clay, further transformed into ferritic soil by total hydrolysis; the two main elements of the resulting soil, iron, and aluminum, are responsible for red/anthracite and blue/purplish colors respectively.
The different shades of color are believed to be a consequence of the molten volcanic rock cooling down at different external temperatures (hence rates), but the causes of their consistent spontaneous separation are yet to be fully clarified.
Another odd feature of the dunes is that they do not seem to erode, despite Mauritius’ heavy and frequent rains. The natural beauty of the site has attracted a healthy tourist trade and a wooden fence was erected to protect the sands, but nothing can protect visitors’ eyes from the striking colors on display.
This phenomenon can also be observed, on a smaller scale, if one takes a handful of sands of different colors and mixes them together, as they'll eventually separate into a layered spectrum.
According to the Wikipedia