🔎 What Is “Selenite” Exactly?

Selenite is the transparent to translucent, well-formed crystal variety of gypsum—the mineral with the formula CaSO₄·2H₂O (calcium sulfate dihydrate). If you’ve ever held a perfectly clear blade that looks like frozen moonlight, that’s selenite. The name nods to Selene, the Greek moon goddess, a poetic fit for a mineral that often appears luminous even in gentle light. (No, it doesn’t actually glow in the dark—if yours does, check for hidden LEDs! 😉)

In the trade, you’ll also hear “satin spar”, a fibrous form of gypsum with a silky sheen, and “alabaster”, a fine-grained, massive gypsum prized for carving. All three are gypsum, but only the glass-clear plates and blades are properly called selenite.

📋 Quick Specs (At a Glance)

💪 Physical Properties (and what they mean in hand)

Hardness & Tenacity

On the Mohs scale, selenite is a gentle ~2. You can scratch it with a fingernail, so treat it like a soft wood finish—no keys, no gritty cloths, and definitely no pocket tumble with quartz or coins. Thin plates can be flexible (fun party trick) but they are not elastic; bend them too far and they’ll remember it… by breaking.

Cleavage & Fracture

Selenite is famous for its perfect cleavage in one direction, allowing it to split into broad, smooth sheets with a silky to pearly sheen. Two additional cleavage directions are present but less perfect. Fracture is uneven to splintery, especially in fibrous material. Those sleek, mirror-like surfaces are a big part of selenite’s visual charm.

Luster & Transparency

Clear selenite often appears vitreous (glassy) and can look almost liquid in good light. On cleavage planes it becomes pearly, and in fibrous material (satin spar) it develops an elegant silky sheen.

Color & Inclusions

Pure selenite is colorless to white. Warm honey or orange hues typically come from iron-rich inclusions, while earthy tones can reflect clay or sand. High clarity is common, but zoning, veils, or fine striations are frequent and can enhance its “moonlit” character.

Weight & Feel

With a specific gravity around 2.30, selenite feels surprisingly light compared to many gemstones. If a “selenite” wand feels unusually heavy and has no obvious cleavage, you may be holding glass instead.

🌈 Optical Behavior (why it looks the way it looks)

Selenite’s beauty is grounded in its simple chemistry and layered structure. Optically, it is biaxial (+), meaning it splits light into two rays traveling at different velocities. In thin pieces the effect is subtle; in thicker plates you can observe birefringence (double refraction) by seeing a faint doubling of edges or text when viewed through the crystal.

Refractive Indices & Birefringence

Typical refractive indices cluster around n_α ≈ 1.519–1.521, n_β ≈ 1.521–1.523, and n_γ ≈ 1.529–1.531. The difference between the fastest and slowest ray—its birefringence—is roughly 0.008–0.010. That’s modest, but enough to produce visible optical effects in hand specimens, and lovely interference colors under polarized light in thin sections.

Under the Microscope

In cross-polarized light, gypsum (selenite) often shows low first-order interference colors—soft grays to first-order white. Because its optical behavior is very well known, thin plates of gypsum are used in microscopy as the classic “λ plate” (first-order red) accessory to analyze mineral orientation and interference figures. In other words, selenite isn’t just pretty—it’s useful optics.

Pleochroism & Dispersion

Selenite is typically non-pleochroic (it doesn’t noticeably change color with orientation), and its dispersion is weak, so you won’t see a rainbow fire like you do in diamond. Its magic is more about clarity, sheen, and the way light softly glides across perfect cleavage surfaces.

Luminescence & UV Response

Pure gypsum is usually non-fluorescent, but some localities produce selenite that does glow under UV due to trace activators or included organics. If your piece lights up, that’s a locality signature rather than a universal trait.

🧩 Varieties & Crystal Habits

• Selenite (strict sense): Transparent, tabular to bladed crystals, often large. Can form dramatic swallowtail twins via repeated twinning.
• Satin Spar: Fibrous, silky-luster bundles or “wands” with chatoyancy. Often sold as “selenite wands” in the trade, though technically it’s the fibrous variety of gypsum.
• Alabaster: Fine-grained, massive gypsum used for carving and sculpture; translucent with a soft glow.
• Desert Rose: Rosette clusters of bladed crystals, typically with sand/clay inclusions giving tan or reddish hues—nature’s way of doing origami with minerals.

Crystal faces may show striations and a distinctive sheen along the length of blades. The forms are governed by the monoclinic structure: layers of CaSO₄ sheets and water molecules connected by hydrogen bonds, which help explain the softness, easy cleavage, and sensitivity to heat and humidity.

🌊 Formation: Where Selenite Grows

Selenite is an evaporite mineral. It forms as saline waters—think lakes, shallow seas, or brines—slowly evaporate, leaving behind layers of sulfate salts. Over time and under stable conditions, gypsum can grow into remarkably large crystals, sometimes meters long in caves or mine environments with warm, mineral-rich waters.

With gentle heating (even modest temperatures), gypsum can partly dehydrate to bassanite (plaster of Paris) and, with further heating and dryness, to anhydrite. This dehydrating–rehydrating cycle is central to gypsum’s industrial uses and also explains why natural selenite is not a fan of hot, dry air streams or direct heat sources.

🧪 Identification & Testing Tips

🧼 Care, Handling & Display

• Keep it dry: Selenite is slightly soluble and can dull in humid or wet conditions.
• No chemical cleaners: Use a soft, dry microfiber cloth. Compressed air (gentle) can remove dust.
• Avoid abrasion: Store away from harder minerals. Line shelves or trays with soft foam or felt.
• Heat caution: Avoid direct heat or prolonged hot sun; dehydration can induce cloudiness or microcracks.
• Mounting: For blades/plates, support along the length; do not point-load the edges.

❓ FAQ

✨ The Takeaway

Selenite’s allure comes from a synergy of structure and light. Its layered monoclinic framework and bonded water give it softness, perfect cleavage, and that unmistakable pearly clarity. Optically, it’s a gentle performer—low refractive indices, modest birefringence, and a knack for looking luminous in simple lighting. In the hand, selenite teaches a lesson every collector eventually learns: soft stones demand soft hands.

Whether you’re drawn to crystal-clear blades, silky satin spar, or desert roses that look like they bloomed from sand, gypsum’s selenite variety offers a quiet, elegant beauty. Handle with care, light it thoughtfully, and it will repay you with that subtle, moonlike glow for years to come.

Final joke: If your selenite ever asks for a spa day, do not offer a water bath. A soft cloth and kind words will do just fine. 😇