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.
Physical & Optical Properties
RI Range1.530–1.543
SG Range2.58–2.64
SG Typical2.61
Hardness (Mohs)6.5–7
Crystal SystemTrigonal
Optic CharacterAggregate
Dispersion0.013
Fluorescence LWWeak white/cream
Fluorescence SWInert
Chelsea FilterInert
PleochroismNone
ColorsMulti, Colorless, Red Pink, Yellow Orange, Brown, Blue Violet, Black
SpeciesQuartz
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)
Price Context
Natural — low ($/ct)$1
Natural — high ($/ct)$50
NotePer carat; common agate is $1–5/ct; fine dendritic, fire, or scenic agate $10–50/ct
Price context is approximate. GemID is not an appraisal tool. Results are indicators, not certified valuations.
Measurement Guides
Identifying an agate? GemID walks through these tests in order — RI, SG, fluorescence, and more.
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