gypsum - sand selenite rose from a dry lake near Oujda, Morocco - hand/display specimen

Selenite gypsum sand rose    CaSO₄ ∙ 2H₂O

Selenite gypsum sand roses form in seasonal desert lakes as they dry. Selenite is typically transparent, but when it crystallizes in sandy conditions it forms rosettes of flattened crystals, the rose “petals,” which are either encrusted with sand or with sand embedded throughout. The color of the gypsum crystals depends on the color of the sand in that dry lake. These Moroccan examples are particularly attractive.

Unlike the white-edged desert roses you see for sale in rock shops, these are unaltered and completely natural. Since gypsum has two attached water molecules, mineral dealers cook selenite roses with a weed burner to drive off the water, turning the edges of the crystals white. They are not found like this naturally, and we prefer that students see this mineral in unaltered form. 

The field photo shows selenite roses from Mexico being burned to make the edges of the flattened crystals white. This is also how plaster is made. Gypsum is heated to above 130˚ Celsius to drive off roughly 75% of the water. When powdered, it becomes plaster. When water is added, the plaster rehydrates to become gypsum. Between two thick paper sheets, it becomes wallboard.

Unburned Mexican sand selenite roses are very difficult to find. Geode slices from Brazil are also altered by dying them in unnatural shades of red or blue, and the unwitting buyer finds them attractive, though they are not what a geology student should see. These Moroccan examples are very good, carefully collected, unaltered, and large.

The pencil is 5 1/2 inches long, for scale.

Select a specimen: You can select a specimen by telling us what is in the photo with it, a black and silver pen, a black mechanical pencil or one of those plus some number of coins, or you can let us make the selection.

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