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

Selenite is the transparent, crystalline variety of gypsum that splits into clear cleavage sheets and blades. Satin spar is the fibrous, silky variety with a cat’s‑eye sheen. They share chemistry but look and behave differently. Retail names blur them, so labeling by texture helps.

Roses & alabaster

Desert rose forms as radiating gypsum plates trap sand into petal clusters; alabaster is fine‑grained, massive gypsum that carves like cold butter and glows softly when lit from within.

Evaporite basins

When saline lakes and inland seas evaporate, dissolved calcium and sulfate meet and settle out as gypsum. Layered deposits can later fracture and regrow as selenite blades in veins and pockets.

Caves & giant crystals

In stable, warm groundwater systems, gypsum can grow extremely slowly into extra‑large selenite crystals. The legendary Cueva de los Cristales (Naica, Mexico) hosts sword‑like crystals reaching many meters.

Salt flats & dunes

On salt flats, selenite blades nucleate just below the surface. Weathering breaks them into glittering gypsum sand—think of brilliant white dune fields made almost entirely of gypsum grains.

Hourglass inclusions

In some sandy flats, selenite traps darker sand along growth zones, creating hourglass shapes inside otherwise clear crystals—a charming growth diary you can hold.

From gypsum to plaster (and back)

Heat drives off part of the water to make plaster of Paris (bassanite, CaSO₄·½H₂O). Add water and it rehydrates and sets—human‑scale mineral alchemy.

Windows of Rome

Thin selenite plates were once used as windowpanes (lapis specularis) in the Roman world—clear enough for light, tough enough for a breeze.

Palette

• Colorless/white — classic selenite plates and satin‑spar silks.
• Honey/amber — from iron or organics trapped during growth.
• Smoky/grey — fine inclusions or irradiation in rare cases.
• Sand‑tinted — desert roses and hourglass selenites with warm inclusions.

Fresh cleavage faces look glassy; fibrous faces of satin spar shimmer with chatoyancy (cat’s‑eye).

Habit words

• Cleavage plates — thin, flexible sheets that split smoothly.
• Blades — elongated crystals with razor‑clean edges.
• Swallowtail twins — V‑shaped twins along a plane, like a bow tie.
• Fibrous ribs — satin spar bundles with silk sheen.
• Rosettes — stacked plates radiating into “petals.”

Photo tip: Use low, raking light (~20–30°). For satin spar, a narrow beam makes the cat’s‑eye “switch on.” For clear plates, back‑light a thin edge for a moon‑glow rim.

Cleavage & twins

At 10×, edges are razor straight where cleavage split. Twin boundaries make neat V’s (swallowtails). Minute fluid or dust trails may parallel growth.

Satin spar’s “silk”

Fibers are parallel and fine. Rotate the piece: a single bright line (cat’s‑eye) glides across the dome, strongest when fibers run along the cab’s length.

Inclusions

Sand grains outline the famous hourglass in some blades; clear plates can hold wispy fluid veils and tiny bubbles. Look for delicate striations along growth.

Calcite

Harder (3), shows rhombohedral cleavage in three directions, and reacts with dilute acid. Many calcites show strong double refraction—selenite does not.

Halite (rock salt)

Cubic cleavage & salty taste (please don’t lick your display case). Halite dissolves rapidly; selenite forms sheets, not cubes.

Celestine & barite

Both are much denser. Barite especially feels heavy (SG ~4.5). Celestine tends pale blue and forms prismatic crystals rather than easy sheets.

Quartz

Hardness 7 (will scratch glass and shrug at a fingernail). Hexagonal prisms and no perfect cleavage—very different handling on the wheel.

“Selenite” wands (mislabel)

Most polished white “selenite wands” are actually satin spar gypsum—fibrous, silky, chatoyant. Beautiful, just label by texture.

Quick checklist

• Fingernail scratch? likely gypsum.
• One superb cleavage plane → sheets.
• No fizz in acid; light in hand.

Cave giants

Naica, Chihuahua (Mexico) — colossal selenite crystals in the “Cave of Crystals,” grown over hundreds of thousands of years in hot, mineral‑rich water.

Salt‑flat specials

Great Salt Plains, Oklahoma (USA) — clear blades with hourglass sand inclusions. A favorite for collectors.

Desert roses

Morocco, Tunisia, Mexico, USA — gypsum rosettes tinted by sand; some look like bouquets forgotten by a very tidy desert.

Window stones of antiquity

Spain & Italy supplied thin selenite plates (lapis specularis) used as early window glazing in the Roman era.

Everyday care

• Keep dry. Avoid soaking, steam, or salty sprays; water dulls edges and can pit surfaces.
• Dust with a soft brush or air bulb; finish with a dry microfiber. (A damp cloth is like rain on a sugar sculpture.)
• Store away from harder stones; gypsum is easily scratched.

Display tips

• Support thin plates along the long edge; avoid pressure across the cleavage plane.
• Use raking light to reveal striations and twins; back‑light alabaster to make it glow.
• Silica gel in cases helps for humid climates.

Lapidary notes

• Hardness 2 = easy shaping, but perfect cleavage demands a gentle touch and abundant coolant.
• For satin spar, orient fibers along the cab’s length for a strong cat’s‑eye.
• Finish with very light pressure; a microcrystalline wax can enhance sheen (optional).

Fingernail test (gentle corner)

Pick an inconspicuous spot: a fingernail should leave a mark. That softness is gypsum’s hallmark and a quick field ID.

Twins in the light

Hold a blade under a desk lamp and rotate. A V‑shaped glow often sweeps along a twin boundary—your very own mineral bow tie.