Agate
Last updated: April 2026
Banded chalcedony variety. Natural agate is typically grey-white with muted banding; vivid commercial colors (red, blue, green) are almost always dyed. Sub-varieties: dendritic agate (fern-like manganese oxide inclusions), moss agate (moss-like inclusions), fire agate (iridescent), and eye agate (concentric rings). Banding pattern is the primary visual identifier.
Agate has a specific gravity of 2.61 and a refractive index of 1.530–1.543, with a Mohs hardness of 6.5–7.0. It is aggregate.
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.
Related: Quartz Varieties
Key Differentiators
- Characteristic concentric or parallel banding in multiple colors — primary visual identifier
- Banding may be wavy, straight, or eye-like (eye agate)
- Waxy luster; hardness 6.5–7
- RI ~1.535 (aggregate reading); SG 2.58–2.64
- Most commercial agate is dyed — natural colors are typically grey-white, red-brown, or muted
Common Simulants
- Glass (banded): Mold-banding in glass is too regular; SG lower (~2.5); isotropic under polariscope; gas bubbles under loupe.
- Synthetic resin / plastic: Much lower SG (< 2.0); warm to touch; isotropic.
Commonly Confused With
Commonly confused with: Lapis Lazuli.
Treatments
- Dyed (extremely common — most commercial agate is dyed)
- Heat Treatment (enhances or alters color)
Measurement Guides
Identifying an agate? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.
Try Loupewise Free →Studying for the GIA, FGA, or FEEG practical? Agate 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 →