SPINEL

Blue spinel and diamond ring

Species:  Spinel
​​Mohs Hardness:  7.5 to 8 out of 10
Color:  Nearly all colors
RI:  1.718 (+0.017 / -0.008) (red and blue spinel often in the high range)
Birefringence:  None
SR/DR/AGG:  SR (rarely with ADR)
Clarity Type:  Type II
Optic Character:  Not applicable
Pleochroism:  None
Spectrum:  Varies (see below)
Fluorescence:  Varies (see below)
SG:  3.60 (+0.10 / -0.03)
Routine Treatments:  None
Additional Enhancements:  Heating

Spinel Facts

Spinel occurs naturally in many beautiful colors.

​Spinel occurs in nearly every color, even colorless.
​
The most valued spinel colors are red, cobalt blue, vivid pink and orange.
 
Fine quality spinel is rare in sizes above 5 carats.

Spinel was cherished by ancient emperors and monarchs, as it was originally thought to be ruby.

In 1783, mineralogist Jean Baptiste Louis Rome de Lisle identified spinel as a different mineral than ruby.
​
​Spinel is an August birthstone.

RED & PINK SPINEL

NATURAL SPINEL

COLOR
Light pink through strongly saturated reddish orange, orangy red, pure red, and purplish red to purple.  (Red spinel's color is caused by chromium, the same trace element that colors ruby and emerald.)

MAGNIFICATION
Minute octahedra or negative crystals, alone or in fingerprint patterns.  Included crystals.  Iron oxide staining.

SPECTRUM
Many red to pink stones show a closely-spaced group of up to 10 lines in the red, with the strongest ones at 684 and 686nm.  There are also lines at 632, 642, 650, 656, 665 and 675nm, plus a broad absorption spectrum of the green area.​

FLUORESCENCE
Weak to strong red or orange under LW.  Inert or weak red or orange-red under SW.


SYNTHETIC SPINEL (FLAME FUSION)

REFRACTIVE INDEX
Red Spinel:  1.722 (+/-0.003), Pink Spinel:  1.728 (+0.012 / -0.008), which are both higher than natural spinel.

COLOR
Pale pink most common.  Red very rare.

MAGNIFICATION
May contain gas bubbles and/or show curved color banding.

SPECTRUM
Same as natural.​

FLUORESCENCE
Pink stones are inert under both LW and SW.  Red stones are strong red under LW and weak to moderate red under SW.

​
SYNTHETIC SPINEL (FLUX)

REFRACTIVE INDEX
1.719 (+/-0.003) which is higher than natural.

COLOR
Usually a vivid, medium dark, slightly purplish red.

MAGNIFICATION
Orangy brown to black flux inclusions, alone or in fingerprint patterns.

SPECTRUM
Usually same as natural, but may show fewer lines in the red area.​

FLUORESCENCE
​Strong purplish red to slightly orangy red under LW and weaker slightly orangy red under SW.

BLUE SPINEL

NATURAL SPINEL

COLOR
Bluish violet through greenish blue, often of low saturation.  Many natural spinels have a lavender color.

MAGNIFICATION
Minute octahedra or negative crystals, alone or in fingerprint patterns.  Included crystals.  Iron oxide staining.

SPECTRUM
Broad band in the blue, centered at 459nm.  Strong, but much narrower, band at 480nm.  Diffused bands at 553, 593 and 632nm.

FLUORESCENCE
Iron type is inert.  Cobalt type is weak to moderate red under LW, inert to SW.


SYNTHETIC SPINEL (FLAME FUSION)

COLOR
Light toned blue resembling aquamarine, greenish blue resembling tourmaline, and vivid, dark, slightly-greenish blue resembling sapphire.

MAGNIFICATION
May contain gas bubbles.

SPECTRUM
Strong bands at 540, 580 and 635nm dominating the green to orange area of the spectrum.  Unlike natural blue spinel, there are no lines or bands in the blue of the spectrum.

FLUORESCENCE
Light Blue:  Weak to moderate orange under LW, chalky blue under SW.

Medium Blue:  Strong red under LW, strong bluish white under SW.

Strong Blue:  Strong red under LW, strong mottled blue under SW.

COLOR FILTER
Strong red


SYNTHETIC SPINEL (FLUX)

COLOR
Usually a saturated, medium dark to dark, very slightly violetish blue.

MAGNIFICATION
Orangy brown to black flux inclusions, alone or in fingerprint patterns.

SPECTRUM
Strong bands between 535 and 550nm, 560 and 590nm, and 615 and 635nm, plus a weak absorption at 430nm that increases through the violet.

FLUORESCENCE
Weak to moderate slightly chalky red to reddish purple, strongest under LW.

COLOR FILTER
Red to orangy red

SYNTHETIC GREEN SPINEL

SYNTHETIC SPINEL (FLAME FUSION)

REFRACTIVE INDEX
1.728 (+0.012 / -0.008)

COLOR
Usually a dark-toned green resembling tourmaline or a light-toned yellowish green resembling peridot.

MAGNIFICATION
Might contain gas bubbles.

SPECTRUM
Generally faint bands at 423nm in the violet and 448.5nm in the blue.

FLUORESCENCE
Light Green Stones:  Strong yellowish green under LW, moderate yellowish green under SW.
​
Yellowish Green Stones:  Strong yellowish green under LW and SW.

Dark Green Stones:  Strong purplish red under LW, strong greenish white under SW.

SYNTHETIC COLORLESS SPINEL

Photo courtesy of EBTH

SYNTHETIC SPINEL (FLAME FUSION)

REFRACTIVE INDEX
1.728 (+0.012 / -0.008)

COLOR
Colorless

MAGNIFICATION
Might contain gas bubbles.

SPECTRUM
Not diagnostic

FLUORESCENCE
​Moderate to strong chalky blue or strong greenish blue under SW, sometimes weak green under LW.