Selenite ≠ satin spar (most “rods” are fibrous)

Selenite is a transparent, crystalline variety of gypsum that cleaves into distinct and blade-like projections. Satin spar is a fibrous, silky variety with a “cat’s eye” effect. Chemistry is the same, but appearance and behavior differ. Retail names often confuse them, so texture-based labeling helps.

Roses and alabaster

Desert rose forms when radially growing gypsum plates trap sand into petal clusters; alabaster is fine-grained, massive gypsum that carves like cold butter and glows softly inside.

Evaporitic basins

When salty lake or inland sea waters evaporate, calcium and sulfate meet and settle as gypsum. Later, layered deposits can crack and regrow selenite blade crystals in veins and cavities.

Caves and giant crystals

In stable, warm underground systems, gypsum can grow very slowly into very large selenite crystals. The legendary Cueva de los Cristales (Naica, Mexico) hides sword-sized crystals reaching many meters.

Salt flats and dunes

In salt flats, selenite blades sprout just below the surface. Weathering breaks them into shiny gypsum sand — imagine dazzling white dunes made almost entirely of gypsum grains.

“Hourglass” inclusions

In some selenite growth zones, it traps darker sand, forming hourglass shapes inside the crystal — a charming growth diary that fits in the palm of your hand.

From gypsum to plaster (and back)

Heating removes some water and produces Paris plaster (basanite, CaSO₄·½H₂O). Adding water rehydrates and hardens it — mineralogy "on a human scale."

Roman windows

Thin selenite plates in the Roman world were used as window glazing (lapis specularis) — transparent enough for light, strong enough for breeze.

Palette

• Colorless/white — classic selenite plates and "silky" satin spar shades.
• Honey/amber — from iron or organic impurities during growth.
• Smoky/gray — fine inclusions or rarely illumination.
• Sand shades — desert roses and "hourglass" selenites with warm inclusions.

Fresh cleavage planes look glassy; satin spar fibers sparkle with "cat's eye" play.

Growth form terms

• Cleavage plates — thin, flexible sheets that split smoothly.
• Blades — elongated crystals with "knife-sharp" clean edges.
• "Swallowtail" twins — V-shaped twins along the plane, like a little frog.
• Fibrous "ribs" — satin spar bundles with a silky sheen.
• Rosettes — stacked plates radially forming "petals."

Photo tip: Low, grazing light (~20–30°). In satin spar a narrow beam "activates" the cat's eye. For transparent plates, illuminate a thin edge from the back — you get a moonlight glow outline.

Cleavage and twins

At 10×, where the plate splits, edges are sharply straight. Twin boundaries form neat V shapes ("swallowtails"). Microscopic fluid or dust threads may run parallel to growth.

Satin spar "silks"

Fibers parallel and fine. Rotate the piece — through the dome one bright line slides ("cat's eye"), strongest when fibers run along the cabochon's length.

Inclusions

Sand grains in some blades highlight the famous "hourglass"; transparent plates may have thin fluid curtains and tiny bubbles. Look for subtle growth lines.

Calcite

Harder (3), shows rhombohedral cleavage in three directions and fizzes with diluted acid. Most calcites have a distinct double refraction — selenite does not.

Halite (rock salt)

Cubic cleavage and salty taste (please do not lick the display). Halite dissolves quickly; selenite forms sheets, not cubes.

Celestine and barite

Both much denser. Barite especially feels heavy (SG ~4.5). Celestine tends to be light blue and grows in prismatic crystals, not light sheets.

Quartz

Hardness 7 (scratches glass and "doesn't underestimate" a nail). Hexagonal prismatic crystals and no perfect cleavage — a completely different behavior on the grinding wheel.

"Selenite" rods (mislabeling)

Most polished white "selenite rods" are actually satin spar gypsum — fibrous, silky, with cat's-eye chatoyancy. Beautiful, just label by texture.

Quick checklist

• Scratches with a fingernail? Probably gypsum.
• One excellent cleavage direction → sheets.
• Does not fizz in acid; light in hand.

Cave giants

Naica, Chihuahua (Mexico) — colossal selenite crystals in the "Cave of Crystals," grown for hundreds of thousands of years in hot, mineral-rich water.

Salt plain features

Great Salt Plains, Oklahoma (USA) — clear blades with sand "hourglass" inclusions. Collector favorites.

Desert roses

Morocco, Tunisia, Mexico, USA — gypsum rosettes painted with sand; some look like bouquets left in a very tidy desert.

Antique window stones

Spain and Italy supplied thin selenite sheets (lapis specularis), used as early window glazing in Roman times.

Daily care

• Keep dry. Avoid soaking, steam, or salty splashes; water smooths edges and can eat away the surface.
• Dust off with a soft brush or air blower; finish with a dry microfiber cloth. (A damp cloth is like rain on a sugar sculpture.)
• Keep separate from harder stones; gypsum scratches easily.

Exposure tips

• Support thin slabs along the long edge; avoid pressure across the cleavage plane.
• Use raking light to highlight folds and twins; backlight alabaster so it glows softly.
• Desiccants (silica gel) in display cases help in humid climates.

Processing notes

• Hardness 2 = easy shaping, but perfect cleavage requires a gentle hand and ample cooling.
• For satin spar, orient fibers along the cabochon's length to strengthen the "cat's eye."
• Finish with very light pressure; microcrystalline wax can improve shine (optional).

Nail test (in a cautious corner)

Choose an inconspicuous spot: a nail should leave a mark. That softness is a quick gypsum identification feature.

Twins in light

Hold the blade under a desk lamp light and rotate. Along the twin boundary, a V-shaped glow often slides — your own mineral frog.