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gypsum - unusual sand selenite from a dry lake in the Atacama Desert, Ica Province, southern Peru - hand/display specimen
gypsum - unusual sand selenite from a dry lake in the Atacama Desert, Ica Province, southern Peru - hand/display specimen gypsum - unusual sand selenite from a dry lake in the Atacama Desert, Ica Province, southern Peru - hand/display specimen gypsum - unusual sand selenite from a dry lake in the Atacama Desert, Ica Province, southern Peru - hand/display specimen gypsum - unusual sand selenite from a dry lake in the Atacama Desert, Ica Province, southern Peru - hand/display specimen gypsum - unusual sand selenite from a dry lake in the Atacama Desert, Ica Province, southern Peru - hand/display specimen gypsum - unusual sand selenite from a dry lake in the Atacama Desert, Ica Province, southern Peru - hand/display specimen gypsum - unusual sand selenite from a dry lake in the Atacama Desert, Ica Province, southern Peru - hand/display specimen gypsum - unusual sand selenite from a dry lake in the Atacama Desert, Ica Province, southern Peru - hand/display specimen

gypsum - unusual sand selenite from a dry lake in the Atacama Desert, Ica Province, southern Peru - hand/display specimen

$ 25.00

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 Peruvian examples were recently discovered and 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 altered specimens are not what a geology student should see.

These unusual and attractive Peruvian examples are very good, carefully collected and unaltered. These were discovered in 2025.

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

 

 

 

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