GEMSTONE FIELD GUIDE
Ruby
Ruby and sapphire are varieties of one mineral species, not separate mineral species.
Ruby crystal in marble, NMNH 176171. Uncut reference specimen; not a faceted gem offered by NumisTrends.
The pink-red corundum crystal is still held in its marble host rock. It has not been shaped into a faceted gemstone.
Photo: Ken Larsen / Smithsonian National Museum of Natural History, CC0. · Source ↗ · CC0 1.0 · resized for the webA verified 3D specimen capture is not available for this entry. No generated stand-in is used.
AT A GLANCE
Start with the evidence.
Confirm the red material first, then investigate growth origin and treatment. Ruby-like optics or a vivid red UV glow do not by themselves establish natural origin.
- GIA — A Case of Mistaken Identity? ↗
- GIA — Synthetic ruby core with flux-grown overgrowth (Fall 2014) ↗
- GIA — Glowing Gems (Winter 2024) ↗
- Refractive index (RI)
- 1.762–1.770
- Specific gravity (SG)
- 3.95–4.05 · typical 4.00
- Mohs hardness
- 9
typical interval
How to read this value
Published reference interval, not necessarily the two readings of every specimen and not an exhaustive identity boundary. Composition, orientation, wavelength, instrument and condition matter.
How to read this value
Bounds expand GIA's 4.00 ± 0.05.
How to read this value
Scratch resistance, not toughness. This ordinal scale is not linear.
These are reference properties, not measurements of the pictured specimen. Reference images are educational—not NumisTrends sale inventory.
See the details and limitations ↓01 · IDENTIFY
Build a profile, not a guess.
Typical material-level reference; not a measurement, identification, origin, treatment determination, or grading result for any specimen.
Refractive index (RI)1.762–1.770
typical interval
Published reference interval, not necessarily the two readings of every specimen and not an exhaustive identity boundary. Composition, orientation, wavelength, instrument and condition matter.
Specific gravity (SG)3.95–4.05 · typical 4.00
Bounds expand GIA's 4.00 ± 0.05.
Mohs hardness9
Scratch resistance, not toughness. This ordinal scale is not linear.
Birefringence0.008–0.010
Typical published birefringence range; do not calculate each specimen's birefringence by subtracting the population RI endpoints.
Optic characterUniaxial negative
CleavageNone
Inclusions, fractures and treatment-related weaknesses may still affect durability.
Open a property for its scope and source. RI and specific gravity have no units. Mohs describes scratch resistance—not resistance to breaking. Do not scratch a stone to test it.
What can look similar?
Red spinel or red garnet
A red appearance can resemble ruby.
How to investigate. Their single refraction contrasts with ruby's double refraction; a trained optical examination can use that distinction.
Color alone is insufficient.
Laboratory-grown ruby
It can show ruby's spectrum, directional color and red fluorescence.
How to investigate. Examine growth structures and use additional laboratory evidence when needed.
A ruby-consistent property result is not a natural-origin result.
02 · TESTS & ULTRAVIOLET
What a test can—and cannot—tell you.
Results depend on the specimen, instrument and method. This is a reference for understanding evidence, not a home authentication or treatment verdict.
UV reaction
- Long-wave UV
- Natural ruby can range from strong red fluorescence to no visible response.
- Short-wave UV
- Natural ruby may show a similar red color more weakly, or remain inert.
Iron can suppress the response. Synthetic ruby often glows strongly, but low-iron natural ruby can also glow strongly; intensity alone cannot separate them.
Optic character and directional color
Differences between ruby and singly refractive red lookalikes.
Reference observation. Ruby is doubly refractive and can show orangey-red and purplish-red directional colors.
Limit. The optic-axis direction can hide the difference; interpretation requires suitable views, not one face-up photograph.
Microscopy of growth structures
Evidence left by natural or artificial crystal growth.
Reference observation. Flame-fusion ruby may show curved growth striae and gas bubbles.
Limit. This is not an exhaustive checklist for every synthetic growth method.
Real laboratory examples
Documented cases, not test results of the reference specimens shown on this page.
A synthetic crystal with two growth histories
GIA documented a flame-fusion ruby core with a flux-grown outer layer. Curved striae and flux-residue features occurred in different regions.
The takeaway. A natural-looking inclusion scene in one area does not settle the identity of the entire specimen.
Use a qualified gemologist for identification work. Do not expose eyes or skin to UV sources, or perform scratch, heat, acid or solvent tests on a gemstone. Do not assume that an absent UV reaction rules a material out.
03 · TREATMENTS & CARE
Protect the stone you actually have.
Disclosed treatments, fractures and settings can change which cleaning methods are appropriate.
Treatment matters despite ruby's durability. GIA advises a damp cloth for fracture-filled, cavity-filled or dyed material; do not assume aggressive cleaning is safe because the host is corundum.
04 · SOURCES & SCOPE
Know where each fact comes from.
Original NumisTrends explanations with primary references. This is not a replacement for gemological training, a laboratory report, or the examination of your own stone.
Properties reviewed: 2026-09-07 · Identification notes reviewed: 2026-09-07
- gia.edu · ruby ↗
- gia.edu · winter 2019 analytical methods geographic origin determination gemstones ↗
- gia.edu · ruby care cleaning ↗
NumisTrends is independent of GIA, Gem-A and the museums cited. A matching property cannot by itself establish identity, natural versus laboratory-grown origin, treatment, geographic origin, grade or value.