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YAG

Last updated: April 2026

YAG (Yttrium Aluminum Garnet, Y₃Al₅O₁₂) is a man-made material with no natural counterpart. It was widely used as a diamond simulant from the 1960s to mid-1970s before being supplanted by cubic zirconia. YAG is still routinely encountered in vintage and estate jewelry. Its fixed RI of 1.833 is instantly diagnostic on a refractometer — no natural gem reads exactly 1.833 except YAG. It is isotropic (like spinel and garnet) with no birefringence. Colored YAG was also produced as simulants for ruby (red/pink), emerald (green), and sapphire (blue) by doping with rare earth elements.

YAG has a specific gravity of 4.55 and a refractive index of 1.833, with a Mohs hardness of 8.0–8.5. It is singly refractive.

Physical & Optical Properties

RI Range1.833
SG Range4.50–4.60
SG Typical4.55
Hardness (Mohs)8–8.5
Crystal SystemCubic
Optic CharacterSR (Singly Refractive)
Dispersion0.028
Fluorescence LWInert
Fluorescence SWInert
Chelsea FilterNot Applicable
PleochroismNone
ColorsColorless, Yellow Orange, Green, Blue Violet, Red Pink
SpeciesSynthetic Garnet
VarietyYttrium Aluminum Garnet
ColorlessYellow OrangeGreenBlue VioletRed Pink

Measured with a polariscope, a refractometer, a hydrostatic balance, a Chelsea filter, a UV torch, a loupe, a darkfield loupe and a spectroscope — step-by-step for each.

Through the refractometer

YAG no reading — over the limit 1.40 1.45 1.50 1.55 1.60 1.65 1.70 1.75 1.80 REFRACTIVE INDEX DORead the scale from bottom to top, then look for theliquid’s own faint line near the top. SEEThe field stays dark, with one faint line near 1.81 thatdoes not move when you change stones. MEANSOver the limit — this stone’s index is above the contactliquid’s own, so the liquid is the last thing the instrumentcan still see. That faint line is your proof the instrumentis working rather than dead. It is a RESULT: it eliminatesevery species below 1.81 in one observation. A field darkwith NO line means something else has gone wrong. Computed from gems.json — edge positions are arithmetic, not drawn. RI increases upward; the field above the boundary is the dark one.

Drawn from the refractive index above, on a 1.40–1.81 scale. How to take this reading.

Through the polariscope

YAG Dark throughout (SR) 0° 45° 90° 135° ROTATION BETWEEN CROSSED POLARS YAG stays dark through the whole rotation. That is singly refractive: there is no second ray to rotate the plane of vibration back into the analyser. Computed from optic character and transparency in gems.json. A polariscope separates singly refractive, doubly refractive and aggregate — it does not show the optic sign.

Drawn from the optic character and transparency above. A polariscope separates singly refractive, doubly refractive and aggregate; the optic sign needs a conoscope. How to take this reading.

Specific gravity, and what it rules out

YAG SG 4.50–4.60 WHERE IT SITS, AND WHAT ELSE IS THERE 1.0 2.0 3.0 4.0 5.0 SPECIFIC GRAVITY 1 other species in the table overlap this range, so a reading here narrows the field to 2 rather than naming the stone. Combine it with a refractive index to separate what is left. Computed from the specific-gravity range in gems.json. Hydrostatic weighing — the bench method this site recommends; heavy liquids are toxic and regulated. Faint ticks are the other species in the table.

Drawn from the specific-gravity range above, against every other species in the table. How to take this reading.

Through the dichroscope

YAG One colour (no pleochroism) IDENTICAL AT EVERY ANGLE YAG shows no pleochroism, so both windows match at every orientation — turning the instrument changes nothing. That is a real negative result, and the reason it is worth knowing is below. Computed from pleochroism and pleochroic colours in gems.json. Window tints are derived mechanically from the colour words, not chosen; a neutral pair depicts no colour at all.

Drawn from the pleochroism above. Two windows that match mean either no pleochroism or the wrong angle — turn the instrument to tell. How the dichroscope compares.

Key Differentiators

Natural vs. Synthetic

Synthetic yag is commercially available (Czochralski (pulled)). Distinguishing natural from synthetic typically requires microscopic examination of internal features.

Loupewise Pro includes a two-phase natural vs. synthetic testing protocol for YAG.

Common Simulants

Commonly Confused With

Commonly confused with: Diamond, Ruby, Sapphire, Emerald.

Measurement Guides

How to Identify a GemstoneHow to Measure Specific GravityHow to Use a RefractometerUV Fluorescence Testing

Identifying a yag? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.

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Studying for the GIA, FGA, or FEEG practical? YAG is one of the 132 species in Loupewise’s drill and exam pool — practice identifying it from its measured properties under timed exam conditions.

Practice exams →