,
Mammoth Cave National Park, Kentucky. Orange color is iron oxide from groundwater.
Crystal habit refers to the shapes that crystals exhibit. Selenite crystals show a variety of habits, but the most common are tabular, prismatic, or acicular (columnar) crystals, often with no imperfections or
inclusions. as at
Santa Sabina in Rome. 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). Gypsum flowers are most often
acicular,
scaly,
stellate, and
lenticular. Gypsum flowers most often exhibit simple
twinning (known as
contact twins); where 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.
Color -gypsum "Buda Rock",
Ray Mine Arizona. A core of glassy and colorless gypsum crystals up to 1 cm in length with a
druse of royal blue azurite, to 0.5 cm in length on the smaller gypsum crystals and included within the larger ones. Gypsum crystals are colorless (most often selenite), white (or pearly – most often satin spar), or gray, but may be tinted brown, yellow, red, or blue by the presence of impurities, such as
iron oxides or
clay minerals.
Transparency Gypsum crystals can be transparent (most often selenite), translucent (most often satin spar but also selenite and gypsum flowers), and opaque (most often the rosettes and flowers). Opacity can be caused by impurities, inclusions, druse, and crust, and can occur in all four crystalline varieties.
Luster Selenite typically shows vitreous
luster, but may show pearly luster on cleavage surfaces. Satin spar shows characteristic silky luster. Luster is not often exhibited in the rosettes, due to their exterior druse; nevertheless, the rosettes often show glassy to pearly luster on edges. Gypsum flowers usually exhibit more luster than desert roses.
Other optical properties Fibrous satin spar exhibits
chatoyancy (cat's eye effect). When cut across the fibers and polished on the ends, satin spar exhibits an
optical illusion when placed on a printed or pictured surface: print and pictures appear to be on the surface of the sample. It is often called and sold as the “television stone” (as is
ulexite). Some selenite and satin spar specimens exhibit
fluorescence or
phosphorescence. ==Occurrence==