Selenite with black raven-feather phantoms from Spain

What is Selenite?

Selenite is the transparent, crystalline variety of the mineral gypsum (CaSO4·2H2O), a hydrated calcium sulfate that typically forms in so-called evaporitic environments. The name comes from the Greek word selēnē (moon), referring to the gentle shimmer of clear gypsum crystals.

The selenite specimens with black phantoms presented here come from northeastern Spain, specifically from the regions of La Rioja and Aragon, and were described in detail by our geologist friend, Jon.

A Glimpse into the Cretaceous Period: The Formation

Selenite crystals often form where calcium- and sulfate-rich water evaporates and the dissolved minerals become concentrated until gypsum crystals grow on the bottom of the lake.

A new collection of Spanish specimens is particularly fascinating: they formed during the Cretaceous period, approximately 120 to 125 million years ago, in shallow evaporative lakes. Since these lakes were rich in preserved organic matter due to oxygen-poor conditions, the selenite crystals grew directly at the interface between sediment and water.

They often formed double-terminated (bipyramidal) shapes and a distinct fishbone habit, indicating extremely rapid growth during strong evaporation. Precisely this rapid crystal growth process was the decisive factor that allowed for the massive inclusion of striking black matter.

Find Locations: La Rioja & Aragon

The selenite crystals originate from Cretaceous sedimentary deposits in northeastern Spain. There are primarily two outstanding localities:

• Villarroya (La Rioja): Provides the most striking specimens with dense black inclusions. Here, both completely black selenites and transparent crystals are found, in which the black phantom patterns are impressively clear.
• Montalbán (Aragon): Produces clear and remarkably large, double-terminated selenite crystals. They often contain fewer, but particularly aesthetic black inclusions. The crystals here reach sizes of up to 20 cm.

Gypsum itself is widely distributed worldwide. However, the combination of excellently formed crystal shapes, black inclusions, and so-called raven-feather phantoms makes these Spanish specimens a unique and fascinating rarity on the market.

The Raven-Feather Phantoms: Tree Rings of Antiquity

The main attraction of the selenites from these two regions are their unmistakable black inclusions. These consist of a combination of organic material, coal, black clay, tiny pyrite crystals, and traces of hydrocarbons, which were incorporated into the mineral during evaporation.

In the specimens from Villarroya, these inclusions are sometimes so dense that the otherwise transparent selenite is completely deep black – while retaining its characteristic vitreous to silky luster surface. Even more remarkable, however, are specimens in which the inclusions run in geometric patterns. Visually, they strongly resemble raven feathers and thus form a visual archive of the crystal's entire growth history.

Each of these internal, black mineral layers reflects changes in the lake environment at the time. It coincides exactly with seasonal evaporation cycles, rainy periods, or stormy weather, through which massive amounts of earth and sediment were periodically washed into the lake.

As a result, the crystal always grew gradually, in layers. Anyone looking at one of these black selenites is looking at an exact prediction of various environmental cycles millions of years ago. These mineral structures can be best compared to tree rings, which also visually highlight annual growth and rainy seasons.

Such growth layers can be analyzed by modern science using stable isotope analysis. For modern geology, they now serve as an important key to decipher past climates and prehistoric environmental changes – and also to understand how the global climate might react in the future.