Hauyne
Last updated: April 2026
Hauyne (also spelled Haüyne) is a rare blue feldspathoid of the sodalite group, found primarily in volcanic phonolite rocks. Gem-quality faceted hauyne is exceedingly rare and sought by collectors; most notable material comes from the Eifel volcanic region of Germany and from Vesuvius, Italy. Its vivid cornflower to royal blue color and extremely low RI (1.494–1.509) and SG (2.44–2.50) are the primary diagnostic characteristics — the RI is so low it falls below the range of a standard refractometer's scale, making it immediately distinguishable from all common blue gem simulants (sapphire 1.762–1.770, blue tourmaline 1.614–1.666, blue topaz 1.609–1.643).
Hauyne has a specific gravity of 2.47 and a refractive index of 1.494–1.509, with a Mohs hardness of 5.5–6.0. 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
- Extremely low RI 1.494–1.509 — below refractometer scale upper limit; far below sapphire (1.762), tourmaline (1.614–1.666), and topaz (1.609–1.643)
- Very low SG 2.44–2.50 — noticeably lighter than sapphire (4.00), tourmaline (3.00–3.20), and topaz (3.49–3.57)
- Isotropic (cubic SR) — no birefringence, no dichroism; sapphire and tourmaline are both anisotropic
- Vivid cornflower to royal blue; often found as small crystals in volcanic matrix
- Exceedingly rare as a faceted gem; primarily a collector stone
Measurement Guides
Identifying a hauyne? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.
Try Loupewise Free →Studying for the GIA, FGA, or FEEG practical? Hauyne 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 →