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Ruby

Ruby • red variety of corundum — Al₂O₃ colored by Cr³⁺ Crystal system: Trigonal • Habit: tabular/prismatic hexagonal forms, barrel crystals, granular Mohs: 9 • SG: ~3.99–4.05 • RI: ~1.762–1.770 • Birefr.: ~0.008–0.010 (uniaxial −) Optics: weak–moderate pleochroism (purplish‑red ↔ orangey‑red); strong red fluorescence in many low‑Fe stones Phenomena: star ruby (6‑ray asterism from rutile “silk”), rare color‑zoning & hexagonal cores

Ruby — Saturated Red, Ancient and Electric

Ruby is corundum tuned to red by chromium. The same chromium that paints it also makes many stones glow under daylight and UV—an inner ember that photographers love and gem cutters chase. Some rubies are crystal‑clear and brilliant; others carry fine “silk” that softens the light into a luminous bloom. Either way, ruby wears red the way a violin wears varnish: deeply.

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What it is
Aluminum oxide (corundum) colored by Cr³⁺. All other colors of corundum are sapphire; only distinctly red corundum is ruby
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Why it captivates
Saturated red with living fluorescent glow, silk that can star, and a history that runs through crowns, talismans, and high jewelry
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Care snapshot
Mohs 9 and tough, but treat fracture‑filled rubies gently (no ultrasonics/steam/harsh acids). Mild soap + water is the safe default

Identity & Naming 🔎

Ruby vs. pink sapphire

Both are chromium‑bearing corundum. When the red is rich and dominant, the gem is called ruby; when it’s lighter in saturation, many labs call it pink sapphire. Boundaries are set by lab standards and can vary by jurisdiction—color is the deciding factor.

Why some rubies “glow”

Chromium absorbs green and blue and emits a red fluorescence. In low‑iron, marble‑hosted rubies this glow intensifies the face‑up color—one reason the classic “pigeon’s blood” look feels electric.

Gem‑cutting nuance: cutters orient rubies to balance pleochroism (bluish‑red vs. orangey‑red) and to manage any color zoning so the table sees the best red.

Where It Forms 🧭

Marble‑hosted rubies

In metamorphic belts, corundum crystallizes in white marble from aluminum‑rich sediments. These rubies are often low in iron and highly fluorescent (Myanmar, Vietnam, Afghanistan).

Basalt‑related rubies

In basaltic terrains and placers derived from them, rubies may be higher in iron with reduced fluorescence and a deeper, sometimes browner red (Thailand, Cambodia, parts of Africa).

Alluvial treasure

Weathered crystals travel into river gravels, where miners recover rounded pebbles—nature’s preform—ready for careful cutting.

Think two main moods: marble = lit from within, basalt = saturated and strong. Both can be beautiful.

Palette & Pattern Vocabulary 🎨

Palette

  • Pure crimson — classic ruby red.
  • “Pigeon’s blood” — vivid red with a subtle cool undertone and strong inner glow.
  • Orangey‑red — warmer axis or zoning.
  • Deep wine — iron‑rich or thicker stones.
  • Silky bloom — scattering from rutile “silk” softens the tone.

Brightness depends on saturation, cut, and fluorescence—well‑cut rubies seem to carry their own pilot light.

Pattern words

  • Silk — oriented rutile needles in three directions (can produce a star in cabochons).
  • Fingerprints — healed fluid inclusions that look like wispy whorls.
  • Color zoning — hexagonal or angular patches of differing saturation.
  • Star lines — intersecting reflective bands in star rubies (6‑ray most common).

Photo tip: For faceted stones, a diffused key light + tiny side kicker reveals color without bleaching. For star rubies, use a single small point light to make the asterism “lock.”

Physical & Optical Details 🧪

Property Typical Range / Note
Chemistry Al₂O₃ (corundum) colored by Cr³⁺; Fe, Ti trace elements influence tone and fluorescence
Crystal system / Habit Trigonal; tabular/prismatic hexagonal forms; granular or metamorphic masses
Hardness (Mohs) 9 (excellent wear resistance)
Specific gravity ~3.99–4.05
Refractive index ~1.762–1.770; birefringence ~0.008–0.010; uniaxial (−)
Pleochroism Weak–moderate: purplish‑red ↔ orangey‑red
Fluorescence Often strong red under LW UV (quenched by Fe)
Phenomena Asterism (6‑ray star) from rutile silk in cabochons; rarely 12‑ray with two silk generations
Treatments Heat (very common), flux‑healed fissures, and lead‑glass filling for heavily fractured material. Beryllium diffusion is more typical in sapphires but can appear—ask for disclosure
Plain‑English optics: chromium sets the color; fine rutile softens it; fluorescence lifts it. Cut ties it all together.

Under the Loupe 🔬

Natural inclusion scenes

  • Rutile silk (three directions, 60° apart).
  • Healed fingerprints and growth zoning (hexagonal).
  • Mineral guests: calcite/dolomite (marble‑hosted), chromite dots (basaltic rubies).

Heat & filling clues

Heat can recrystallize silk into snowball pinpoints; flux healing leaves glass‑like “feather” with flux residue; lead‑glass filling shows gas bubbles and blue flashes along surface‑reaching fissures.

Star setup

Cabochons cut with silk aligned parallel to the base show a 6‑ray star that tracks the light source. A slightly higher dome often yields a crisper star.

Look‑Alikes, Labs & Imitations 🕵️

Natural “confusers”

  • Red spinel — singly refractive (no double refraction), RI ~1.718, no pleochroism.
  • Red garnet — singly refractive, higher RI/SG, often browner; no pleochroism.
  • Rubellite tourmaline — stronger pleochroism, RI ~1.62–1.65, different crystal habit.

Lab‑grown ruby

Flame‑fusion (Verneuil) shows curved growth lines and occasional gas bubbles; flux‑grown rubies have wispy flux “fingerprints” and metallic platelets; hydrothermal types are rarer. Lab rubies are corundum—just disclose origin.

Glass & assembled

Red glass has low RI, often bubbles and a “soft” look. Doublets/triplets may sandwich thin ruby between glass—watch for a join line at the girdle.

Quick checklist

  • RI ~1.77 with birefringence and weak pleochroism?
  • Chromium spectrum/fluorescence present?
  • Natural inclusions vs. curved lines/bubbles? → Decide natural vs. lab.

Localities & Stories 📍

Where it shines

Myanmar (Mogok) — famed fluorescent reds from marble. Mozambique — abundant modern production with rich, often slightly purplish reds. Sri Lanka — silky stars and bright reds from gravels. Thailand/Cambodia — deep, iron‑richer stones. Also Vietnam, Tajikistan, Afghanistan, Madagascar, and Tanzania.

How people use it

Faceted solitaires and halo rings, luminous star cabochons, and bead strands from translucent material. The octagonal “emerald cut” in ruby? Absolutely—step cuts can play up rich color in clean crystals.

Labeling idea: “Ruby (corundum) — natural/lab‑grown — treatment (none/heat/flux‑healed/lead‑glass) — cut — locality.” Clear, honest, complete.

Care & Jewelry Notes 🧼💍

Everyday care

  • Mild soap + lukewarm water; soft brush under the stone; rinse and dry.
  • Avoid ultrasonics/steam/strong acids for fracture‑filled or heavily included stones.
  • Store separately—ruby can scratch most gems, and diamond can scratch ruby.

Setting guidance

  • Sturdy prongs or protective bezels for daily rings.
  • Brilliant cuts maximize sparkle; step cuts emphasize color and clarity.
  • Yellow gold warms the red; white metals cool it—choose the mood you want.

Star ruby tips

  • Dome height and silk density control star sharpness.
  • Single point light for display; diffuse light softens or hides the star.
  • Keep cabochons free of film—tiny residues mute asterism quickly.
Display tip: Place a faceted ruby beside a star cabochon on a rotating turntable. Two personalities, one chemistry—visitors remember the comparison.

Hands‑On Demos 🔍

UV glow check

Under long‑wave UV, many rubies light up red. Compare a marble‑hosted ruby (usually bright) with an iron‑rich stone (often quieter)—instant geology lesson.

Dichroscope peek

View through a dichroscope to see two shades of red from pleochroism. Rotate the stone to watch them trade places.

Ruby is color with a pulse—chromium’s heartbeat, cut and set to shine.

Questions ❓

Is lab‑grown ruby “real”?
It’s chemically ruby (corundum) grown in a lab. Inclusion scenes differ from natural. Both are beautiful when clearly labeled.

What is “pigeon’s blood”?
A trade description for a vivid, slightly cool red that often shows strong fluorescence and lively brightness. It’s about look, not a single formula.

Are most rubies treated?
Many are heat‑treated to refine color/clarity. Some fractured material is flux‑healed or lead‑glass filled. Good labels say which, so you can care accordingly.

Can ruby be worn every day?
Yes—Mohs 9 with good toughness. Just mind the treatment status and store it separately from other gems.

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