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
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
Drawn from the refractive index above, on a 1.40–1.81 scale. How to take this reading.
Key Differentiators
- RI 1.833 — fixed, single value (isotropic); immediately diagnostic
- SG 4.55 — noticeably heavy for apparent size
- Singly refractive — isotropic; no birefringence, no facet doubling
- Hardness 8–8.5 — does not scratch easily
- Historic diamond simulant from 1960s–70s; now replaced by CZ but still found in vintage jewelry
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
- Cubic Zirconia (CZ): CZ replaced YAG as the dominant diamond simulant after ~1976. Both are isotropic synthetics with high luster. CZ has much higher SG 5.6–6.0 (vs YAG 4.50–4.60); CZ RI ~2.15–2.18 (vs YAG ~1.833). A thermal probe reads both as non-diamond; use SG and RI to confirm which synthetic it is.
- Almandine Garnet: YAG (yttrium aluminum garnet) was designed to mimic natural garnet. Almandine has RI 1.770–1.830 (below YAG's 1.833) and SG 3.93–4.30 (below YAG's 4.50–4.60). Natural garnet shows inclusions; YAG is flawless under magnification with no natural growth features.
- Synthetic Spinel: Synthetic spinel (especially colored varieties) is another synthetic simulant sometimes confused with colored YAG. Synthetic spinel SG ~3.63 (much lower than YAG ~4.55); RI ~1.718 (lower than YAG ~1.833). Distinguishable by refractometer and heft test.
Measurement Guides
Identifying a yag? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.
Try Loupewise Free →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 →