Selenite, satin spar, desert rose, and gypsum flower are four varieties of gypsum; all four varieties show obvious crystalline structure. The four "crystalline" varieties of gypsum are sometimes grouped together and called selenite.
All varieties of gypsum, including selenite and alabaster, are a very soft mineral (hardness: 2 on Mohs Scale) composed of calcium sulfate dihydrate (meaning has two molecules of water), with the chemical formula CaSO4·2H2O.
The most important identifying characteristic is how soft gypsum is, as any variety of gypsum can be easily scratched with a fingernail. Also because gypsum has natural insulating properties, all varieties feel warm to the touch.
Since the late 20th century, with the growing interest in crystal therapy and crystal healing in the New Age, Neo Pagan, and alternative healing countercultures, the four crystalline varieties of gypsum have increased in popularity and commercial value. This increased interest has translated itself into both the retail mineral and jewellery trades. In the retail mineral trade, all four crystalline varieties are offered as rough, carved, or tumbled specimens. In the retail jewellery trade, selenite crystals with interior druse are offered as a form of drusy jewellery.
Selenite crystals commonly occur as tabular, reticular, and columnar crystals, often with no imperfections or inclusions; and thereby, can appear water or glass-like. Many collectible selenite crystals have interesting inclusions such as, accompanying related minerals, interior druse, dendrites, and fossils. In some rare instances, water was encased as a fluid inclusion when the crystal formed (see Peñoles Mine reference in external links).
Selenite crystals sometimes forms in thin tabular or mica-like sheets; and has been used as glass panes.
Selenite crystals sometimes will also exhibit bladed rosette habit (usually transparent and like desert roses) often with accompanying transparent, columnar crystals. Selenite crystals can be found both attached to a matrix or base rock; but can commonly be found as entire free-floating crystals, often in clay beds (and as can desert roses).
Satin spar is almost always prismatic and fibrous in a parallel crystal habit. Satin spar often occurs in seams, some of them quite long; and is often attached to a matrix or base rock.
Desert roses are most often bladed, exhibiting the familiar shape of a rose, and almost always have an exterior druse. Desert roses are almost always unattached to a matrix or base rock; most often found lying around.
Gypsum flowers are most often acicular, scaly, stellate, and lenticular. Gypsum flowers most often exhibit simple twinning (known as contact twins); where that parallel, long, needle-like crystals, sometimes having severe curves and bends, will frequently form “ram’s horns”, "fishtail", "arrow/spear-head", and "swallowtail" twins. Selenite crystals can also exhibit “arrow/spear-head” as well as “duck-bill” twins. Both selenite crystals and gypsum flowers sometimes form quite densely in acicular mats or nets; and can be quite brittle and fragile. Gypsum flowers are usually attached to a matrix (can be gypsum) or base rock.
It is not recommended that you leave any form or variety of gypsum in water - as extended submersion will either dissolve or degrade the mineral. Detergents and soaps should be avoided, as they affect luster, particularly with selenite and satin spar - even lukewarm water can affect luster of selenite and satin spar.
When cut across the fibers and polished on the ends, satin spar exhibits an optical illusion when placed on a printed or pictured surface; and is often called and sold as the “television stone” (as is ulexite). Print and pictures appear to be on the surface of the sample.
All four crystalline varieties are slightly flexible, though will break if bent significantly. They are not elastic, meaning they can be bent, but will not bend back on their own.
All four crystalline varieties are sectile in that they can be easily cut, will peel (particularly selenite crystals that exhibit mica-like), and like all gypsum varieties, can be scratched by a fingernail (hardness: 2 on Mohs Scale). The rosettes are not as quite soft due to their exterior druse; nevertheless, they too can be scratched.
Selenite crystals that exhibit in either reticular or acicular habits, satin spar, in general (as fibrous crystals are thin and narrow), desert roses that are thinly bladed, and gypsum flowers, particularly acicular gypsum flowers, can be quite brittle and easily broken.
Gypsum is formed as an evaporative mineral, frequently found in alkaline lake muds, clay beds, evaporated seas, salt flats, salt springs, and caves. Gypsum, also, is frequently found in conjunction with other minerals such as, copper ores, sulfur and sulfides, silver, iron ores, coal, calcite, dolomite, limestone, and opal. Gypsum has been dated to almost every geologic age since the Silurian Period 443.7 ± 1.5 Ma.
In dry, desert conditions and arid areas, sand may become trapped both on the inside and the outside of gypsum crystals as they form. Interior inclusion of sand can take on shapes such as, an interior hourglass shape common to selenite crystals of the ancient Great Salt Plains Lake bed, Oklahoma, USA. Exterior inclusion (druse) occurs as embedded sand grains on the surface such as, commonly seen in the familiar desert rose.
When gypsum dehydrates severely, anhydrite is formed. If water is reintroduced, gypsum can and will reform - including as the four crystalline varieties. An example of gypsum crystals reforming in modern times is found at Philips Copper Mine (closed and abandoned), Putnam County, New York, USA where selenite micro crystal coatings are commonly found on numerous surfaces (rock and otherwise) in the cave and in the dump.
Whereas geology, mineralogy, and rockhounding groups, clubs, and societies as well as museums usually date (of find and geologic), photograph, and note location of minerals, much of the retail mineral and jewellery trade can be somewhat casual about dates, locations, and descriptive claims.
Giant gypsum crystals and gypsum speleothems sites (mainly selenite crystals):