About two weeks ago, I posted a photo of a Paris restaurant called La rose des Sables where we had a fine Moroccan-style meal (des tagines) years ago. I happen to have a small rose des sables — called a "desert rose" in English, according to Wikipedia — that I believe I bought (I didn't just find it...) in an outdoor market in the sub-Saharan city of Niamey, which is the capital of the country of Niger. It was March 1985 and I was traveling as a member of the press pool covering a trip that then-vice-president George H.W. Bush was leading to African countries during the devastating drought and famine that the area was suffering through back then.
Here's how Wikipédia describes the rock formation called une rose des sables: [c'est] une roche évaporitique formée par la cristallisation lenticulaire de minéraux solubles, et dont la disposition rappelle les pétales d'une rose. In English, Wikipedia says: "Desert rose is the colloquial name given to rose-like formations of crystal clusters of gypsum or baryte which include abundant sand grains. The 'petals' are crystals flattened on the crystallographic axis, fanning open in radiating flattened crystal clusters."