GEMSTONE FIELD GUIDE
Spinel
This record describes ordinary gem spinel; other spinel-group compositions and laboratory-grown material can have different constants.
Pink spinel, NMNH G10630. Museum reference photograph; not NumisTrends inventory.
This pink stone is cataloged as spinel, not ruby. Similar colors can occur in different gem minerals.
Photo: Chip Clark / 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.
Gem spinel is a distinct mineral, not a kind of ruby. Establishing spinel identity does not by itself establish natural growth or absence of treatment.
- Refractive index (RI)
- Approximately 1.718 typical; some compositions rise to approximately 1.740
- Specific gravity (SG)
- 3.58–3.61 · typical 3.60
- Mohs hardness
- 8
typical value with compositional extension
How to read this value
GIA gives 1.718. Gem-A gives about 1.717 and notes higher RI in chromium-rich red stones and zinc-bearing blue material. This is not one stone's birefringence range.
How to read this value
GIA typical value and Gem-A's explicit ordinary-spinel interval; do not apply this interval automatically to every spinel-group composition or synthetic spinel.
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)Approximately 1.718 typical; some compositions rise to approximately 1.740
typical value with compositional extension
GIA gives 1.718. Gem-A gives about 1.717 and notes higher RI in chromium-rich red stones and zinc-bearing blue material. This is not one stone's birefringence range.
Specific gravity (SG)3.58–3.61 · typical 3.60
GIA typical value and Gem-A's explicit ordinary-spinel interval; do not apply this interval automatically to every spinel-group composition or synthetic spinel.
Mohs hardness8
Scratch resistance, not toughness. This ordinal scale is not linear.
BirefringenceNone (isotropic)
Do not interpret the compositional RI variation as birefringence.
Optic characterIsotropic / singly refractive (cubic)
CleavageNone
No cleavage in Gem-A's gem-spinel description; fractures and inclusions may still affect cleaning safety.
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?
Ruby
Red spinel and ruby can have similar color.
How to investigate. A gemologist compares refraction and directional color: ruby is doubly refractive, unlike normal spinel.
Viewing direction matters; color alone is insufficient.
Laboratory-grown spinel
Synthetic spinel can resemble natural spinel; it is laboratory-grown material, not merely a different red stone.
How to investigate. Microscopy may reveal flux residues or growth features; difficult cases require spectroscopic follow-up.
No visible inclusion is not a natural-origin conclusion.
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
- GIA's 2014 dark pink-purple, 26.73 ct spinel showed very weak red longwave fluorescence.
- Short-wave UV
- That same specimen was inert under shortwave UV.
This is one heated natural spinel, not a universal spinel response or an origin test. UV is supporting evidence for trained, protected examination, not a home authenticity verdict.
Refractive index and optic character
Whether the optical profile fits spinel rather than visually similar corundum.
Reference observation. Spinel is normally singly refractive; ruby and sapphire are doubly refractive.
Limit. RI varies with composition. A gemologist checks more than one optical clue; a matching reading is not proof of natural growth.
Microscopy with photoluminescence follow-up
Growth features, fracture filling and possible heating.
Reference observation. Some heated spinels retain little microscopic evidence; laboratory spectra can supply additional information.
Limit. An apparently unchanged inclusion does not always exclude heating. Its mineral identity matters, as demonstrated by GIA's spinel-in-spinel case.
Real laboratory examples
Documented cases, not test results of the reference specimens shown on this page.
An unchanged inclusion in heated spinel
GIA found a nearly invisible inclusion consistent with spinel inside a spinel host. Despite its unaltered appearance, combined laboratory evidence supported heating.
The takeaway. A same-material inclusion may survive with the host; assess the whole evidence set before declaring a stone unheated.
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.
Use gentle warm-soapy-water cleaning. High heat can fade some colors; fractures and filling change cleaning suitability. Fracture-filled spinel should receive only warm soapy water or a damp cloth, not automatic steam or ultrasonic cleaning.
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
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.