Sapphire vs. ruby

Both are corundum (Al₂O₃). When chromium turns the stone red, we call it ruby. All other hues are sapphire: blue is the classic; the rest are “fancy sapphires.”

Where the colors come from

Blue: intervalence charge transfer between Fe²⁺ and Ti⁴⁺ gives that saturated cobalt‑to‑royal tone. Yellow/green: Fe³⁺. Pink/purple: Cr³⁺ ± Fe. Color change: V and Cr working together to shift daylight vs. incandescent balance. Padparadscha: a delicate pinkish‑orange blend—lotus‑flower inspired.

Metamorphic belts

In aluminum‑rich, silica‑poor rocks (gneiss, schist, marble), corundum crystallizes during regional metamorphism. These can yield lighter, “open” blues and fine fancy hues.

Basaltic terrains

Corundum crystals travel up with alkali basalts and weather into alluvial gravels. Basalt‑related sapphires commonly show iron‑rich, deeper or inky blues and teals.

After the mountain‑making

Because sapphire is tough, many deposits are placer: rivers concentrate durable corundum, zircon, and spinel into gemmy streaks—nature’s treasure traps.

Star setup

Upon slow cooling, tiny rutile exsolve along crystallographic directions, weaving three sets of needles 60° apart. Light reflects along these, giving a star when cabbed.

Zoning happens

Growth in pulses and shifting chemistry draw hexagonal color zones or “bullseyes.” Cutting orientation can either hide these or show them off as geometry.

Treatments you’ll meet

Gentle to high‑temperature heat treatment is common to clarify and balance color. Diffusion (esp. beryllium for orange/pink) can alter hue more dramatically; a minority are filled to heal fractures. Clear labeling keeps collections useful.

Palette

• Cornflower / royal blue — the classic, from bright to deep.
• Teal / green‑blue — Fe‑rich or Montana flavors.
• Yellow — sunny to gold.
• Pink / purple — Cr‑tinted romance.
• Padparadscha — tender pinkish‑orange “lotus.”
• Grey / near‑colorless — subtle, modern tones.

Blue sapphires often show pleochroism (blue ↔ slightly greenish‑blue or violet‑blue) depending on orientation and light.

Pattern words

• Silk — clouds of fine rutile needles; can soften color to a velvety glow.
• Hex zoning — concentric hexagons or sector bars of color.
• Asterism — six (sometimes 12) rays on a dome; sharpest with abundant, oriented silk.
• Color‑change — blue‑green by day, purple by night (varies with chemistry).

Photo tip: For blue, use a neutral light and a dark card underneath; for stars, a single point light above the dome makes the rays snap into place.

Silk & stars

Fine, parallel rutile needles in three directions at 60° can make asterism. In faceted stones, faint silk adds a soft “sleepy” texture that can be beautiful.

Classic inclusions

Zircon crystals with stress halos, hexagonal growth lines, fingerprint heals, and negative crystals are all friendly signs of nature’s hand.

What hints at lab or heavy diffusion

Flame‑fusion synthetics show curved growth lines and gas bubbles; surface or lattice diffusion can show strong color along facet junctions and at the surface with paler cores. Magnification tells great stories.

Blue spinel

Lower RI (~1.718), singly refractive (no pleochroism), often slightly softer look. A dichroscope quickly separates it from pleochroic sapphire.

Iolite (cordierite)

Strong pleochroism (blue‑violet ↔ pale/yellowish) but softer (Mohs ~7–7.5) and more brittle; different crystal system and inclusions.

Blue topaz / tanzanite

Topaz has perfect basal cleavage and RI ~1.61–1.62; tanzanite shows trichroism (blue‑violet‑burgundy) and is softer. Neither hits sapphire’s hardness 9.

Glass & doublets

Glass has low RI, bubbles, and no pleochroism. Doublets show join lines under magnification—look around the girdle.

Kyanite & blue zircon

Kyanite shows two very different hardnesses and perfect cleavage; zircon has high birefringence (facet “doubling”) and is denser.

Quick checklist

• Hardness 9 (resists scratches from quartz/steel).
• Pleochroism present? Good sign for sapphire.
• No curved striae; inclusions feel “geologic,” not bubbly.

Blue icons

• Kashmir (historic) — velvety “cornflower” blues from abundant fine silk.
• Sri Lanka (Ceylon) — lively medium blues and a full rainbow of fancy colors.
• Myanmar (Mogok) — rich royal to inky blues, fine pinks too.
• Madagascar — major modern supply; wide palette including pad‑like pastel blends.

Distinctive characters

• Australia — deep, sometimes inky blues (Fe‑rich basaltic lineage).
• Montana, USA — bright teals, greens, and Yogo’s famously clean medium blues.
• East Africa (Tanzania/Kenya) — Umba/Tunduru produce yellow, pink, and color‑change surprises.
• Cambodia/Thailand — classic blues; Chanthaburi also a major cutting/treatment hub.

Everyday care

• Clean with lukewarm water + mild soap + soft brush; rinse & dry.
• Ultrasonic/steam: generally fine for unfilled, heat‑treated stones; avoid on filled or heavily included pieces.
• Hardness 9 ≠ invincible—sharp blows can exploit parting. Treat like a fine lens.

Jewelry tips

• Blue shows best in well‑proportioned cuts that avoid “windows.”
• For stars, cut a high dome with the c‑axis centered so the star sits dead‑center.
• Fancy colors (yellow, pink) shine in warm metals; blues love contrast with white metals.

On the wheel

• Orient to manage pleochroism—aim for the richer axis face‑up.
• Pre‑polish through 3k–8k → finish with alumina or diamond on a firm pad.
• Mind parting planes when sawing; a tiny tilt can save a big chip.

Pleochroism peek

Hold a blue sapphire and rotate under neutral light. With a simple dichroscope (or polarized sunglasses), you’ll see two shades—often blue and slightly greenish‑blue/violet‑blue—trading places.

Star test

Shine a single penlight above a star cabochon. A six‑rayed star should glide smoothly. If it splits or wanders, the dome may be off‑axis—or you’ve discovered a quirky double‑star!