Iolite: properties, varieties and value factors

Key properties

Chemical formula	Mg2Al4Si5O18
Crystal system	Orthorhombic
Mohs hardness	7-7.5
Toughness	Fair
Refractive index	1.542-1.551
Birefringence	0.0080-0.0120
Optic character	Biaxial negative (sign can vary near indialite) (-)
Dispersion	0.0170
Specific gravity	2.56-2.66
Cleavage	Distinct in one direction ({010})
Fracture	Uneven to conchoidal; brittle
Habit	Short pseudo-hexagonal prisms (due to twinning), waterworn pebbles, massive
Lustre	Vitreous; polish: Vitreous
UV fluorescence	Inert
Pleochroism	Signature property - strong trichroism: colourless to pale yellow, blue, and violet to violetish blue in three crystal directions.
Cause of colour	Iron (Fe2+) in the cordierite structure produces the violetish blue; the same iron drives the striking pleochroism.
Typical inclusions	Colour zoning aligned with pleochroism directions, liquid feathers, plates of hematite (bloodshot iolite), mica flakes; metamict-like disc cracks in some pebbles
Stability	Stable to light; fusible under torch; attacked by acids
Heat sensitivity	Low in normal use but fuses under jeweller's torch
Care and cleaning	Warm soapy water safe; ultrasonic and steam risky
Birthstones	Suggested gift for the 21st wedding anniversary
Availability	Limited but regular supply of small to medium faceted stones
Recognition	Face-up violetish blue with a colourless to straw side colour when viewed through the girdle is nearly diagnostic; confirm with RI 1.54-1.55, biaxial negative figure, inert LW UV and SG about 2.6.

Varieties

Iolite

Colour	violetish blue through slightly violetish blue
Transparency	transparent to translucent
Clarity type	Type II (typically included)
Phenomena	Rare asterism, chatoyancy and aventurescence (bloodshot iolite with hematite plates)
Appearance	A smoky, sapphire-like violetish blue face-up that washes to near-colourless straw when viewed from the side - the famous 'water sapphire' effect; best stones show even, saturated blue without grey.
Trade names	dichroite • cordierite • Vikings' compass (legendary sun-compass stone) • bloodshot iolite (aventurescent)
Misnomers	water sapphire • lynx sapphire (both improper sapphire references)
Typical sizes	1 to 10 ct usual; clean stones above 10 ct rare
Cutting styles	Faceted (must be oriented so the blue direction faces the table); cabochons when chatoyant or included
Key separations	vs sapphire: sapphire RI 1.76-1.77, uniaxial -, SG 4.0; vs tanzanite: tanzanite RI 1.69-1.70, SG 3.35; vs indicolite: tourmaline DR 0.014-0.032 and different pleochroism; vs amethyst: amethyst uniaxial with weaker dichroism
Sources (localities)	India (main) • Sri Lanka • Madagascar • Tanzania • Brazil • Myanmar • Namibia • Norway • Canada (British Columbia) • USA (Wyoming)
Availability	Limited but steady; calibrated melee and one-off larger gems
Market notes	An affordable sapphire substitute look; the side-view colour loss is the key customer education point; 21st anniversary gift
Care notes	Warm soapy water; ultrasonic and steam risky; remove before torch work
In short	Iolite delivers a smoky violetish blue at gentle prices, defined by trichroism that turns the stone colourless when viewed sideways. Sapphire-like at a glance, it is instantly separated by its low RI and SG.

Size (ct)	USD per carat	Tier	As of
1.00–5.00	$8.00 – $50.00	commercial to fine	2026-10-26
5.00–10.00	$20.00 – $100.00	fine	2026-10-26

Retail market survey, per carat unless noted. Ranges describe what commercial to fine stones trade at; exceptional stones and provenance premiums sit above them.

Sources

GIA gem encyclopedia · mindat.org · Wikipedia

Data compiled and fact-checked against gemmological literature, October 2026 (JDMIS gem reference). Properties are ranges accepted in gemmological practice; individual stones vary.

Key properties

Varieties

Iolite

Sources

See also