Anhydrite, the “dry” sulfate

Anhydrite is calcium sulfate without water in its crystal structure (CaSO₄). Its hydrated counterpart is gypsum (CaSO₄·2H₂O). Angelite is a lapidary nickname for the blue, fine‑grained massive variety used for carvings and cabochons.

Why the blue?

The pastel blue comes from trace impurities (and very fine scattering) within the otherwise colorless sulfate. Angelite often shows white calcite veins and occasional rusty iron specks that add gentle character.

Evaporite chemistry

When seawater or saline lake water evaporates in stages, different salts crystallize out. Calcium sulfate is one of the earliest minerals to precipitate. At higher temperatures and lower water activity, it forms anhydrite; under cooler, wetter conditions, it forms gypsum.

Gypsum ↔ anhydrite

In nature the two can transform back and forth. Burial, heating, or drying can convert gypsum to anhydrite; influx of water can hydrate anhydrite into gypsum, expanding slightly—one reason to keep angelite dry.

Massive textures

Angelite typically occurs as massive beds or nodular zones rather than showy crystals. The fine grain and even color make it attractive for smooth, matte polishes and softly rounded forms.

Palette

• Powder to cornflower blue — classic angelite tone.
• Chalk‑white — thin calcite or gypsum veining, soft marbling.
• Cool grey — subtle mottling where grains vary in size.
• Rust freckles — tiny iron oxide pinpoints or streaks.

The finish is usually silky‑matte to softly glossy; angelite seldom reaches a glassy polish like quartz, and that’s part of its charm.

Pattern words

• Cloud wash — even fields of blue with slight tonal drift.
• Feather veining — hairline white calcite threadlets.
• Snowflakes — tiny pale spots where gypsum/calcite blooms.
• Patchwork — fine‑grained blocks with barely visible seams.

Photo tip: Use soft, diffuse light. A harsh spotlight can gloss over the velvet surface; window light at ~30° keeps the blue true and shows gentle veining.

Cleavage geometry

At 10×, angelite breaks into blocky, right‑angled chips. Tiny, parallel cleavages reflect pearly light; surfaces feel smooth but slightly “buttery.”

Calcite threads

Hairline white veins often cut the blue ground. A pin‑head of cold dilute acid on a scrap will fizz on these (calcite) but not on the surrounding anhydrite.

Gypsum blooms

On pieces exposed to humidity, you may see powdery pale patches (incipient gypsum). They buff off but indicate the stone wants drier air.

Blue calcite

Similar color but shows rhombohedral cleavage (slanted, not right‑angled). Calcite is lighter (SG ~2.7) and fizzes broadly in acid; angelite doesn’t (except tiny calcite veins).

Celestine (celestite)

Pale blue strontium sulfate—often forms crystals and feels much heavier (SG ~3.9–4.0). Cleavage fragments aren’t strictly rectangular like anhydrite’s.

Blue chalcedony

Waxy luster and hardness 7 (resists a knife); conchoidal fracture rather than blocky cleavage. Translucence is more even and glow‑like.

Larimar / hemimorphite / smithsonite

These can share pastel blues but show botryoidal or fibrous textures and different heft/hardness. Angelite is fine‑grained, velvety, and cleaves in neat rectangles.

Howlite (dyed)

Typically white with grey veining and often dyed blue. Under magnification dyes pool in pores; angelite’s color is intrinsic and even.

Quick checklist

• Right‑angle cleavage chips?
• Soft (knife scratches) but feels heavier than calcite?
• No broad fizz in acid? (Only veins fizz.)

Classic source

Peru popularized blue anhydrite in the gem world; much of the material sold as “angelite” was quarried from evaporite beds in the Andes and shaped into beads, cabs, and carvings.

Also encountered

Blue to grey anhydrite occurs in evaporite sequences worldwide (e.g., parts of Mexico, the Mediterranean basin, and other arid‑basin deposits). Color and texture vary by locality and grain size.

Everyday care

• Keep dry. Avoid soaking, steam, and bathrooms with constant humidity.
• Clean with a dry microfiber or a barely damp cloth followed by immediate drying.
• Store away from harder stones (quartz dust can scuff the satin finish).

Jewelry guidance

• Best as pendants, earrings, brooches. For rings/bracelets, use protective bezels and gentle wear.
• Avoid exposure to perfumes, chlorinated water, and long sun‑heat cycles that invite micro‑cracking.

On the wheel

• Work cool with light pressure; support thin areas—cleavage is perfect in three directions.
• Pre‑polish through 1k–3k → finish with alumina or cerium on a soft pad for a satin glow (not glassy).
• Consider a micro‑bevel around rims to reduce chipping; some artisans apply a thin, reversible micro‑crystalline wax to help repel moisture.

Cleavage corners

Examine an offcut: tiny chips tend to be rectangular with right angles—an anhydrite signature that sets it apart from conchoidal stones like chalcedony.

Heft test

Compare similar‑sized pieces of blue calcite and angelite in your hands. Angelite’s higher SG usually feels a shade denser—subtle, but noticeable with practice.