Gem library
Sodalite
Blue marbled with white, lapis's cousin
Sodalite is a violet-blue feldspathoid, very often marbled with white veins. Pyrite-free and lighter than lapis lazuli, it is its commonest substitute — and its hackmanite variety has an astonishing property: it changes colour under UV.
- Hardness (Mohs)
- 5.5 – 6
- Density
- 2.14 – 2.40
- Crystal system
- Cubic
- Refractive index
- 1.483 – 1.487
- Cleavage
- Distinct on {110}, imperfect
Mineralogical identity
- Family
- Feldspathoids — sodalite group
- Species
- Sodalite
- Chemical formula
- Na₈(AlSiO₄)₆Cl₂
- Crystal system
- Cubic
- Classification
- Semi-precious stone (ornamental rock)
- Rarity
- Very common
- Indicative price
- €0.20 – 3/ct
Etymology
From sodium, which it contains in abundance, and the Greek lithos, stone.
Physical & optical properties
- Hardness (Mohs)
- 5.5 – 6
- Density
- 2.14 – 2.40
- Refractive index
- 1.483 – 1.487
- Birefringence
- None
- Cleavage
- Distinct on {110}, imperfect
- Fracture
- Conchoidal to splintery
- Lustre
- Vitreous to greasy
- Diaphaneity
- Translucent to opaque
- UV fluorescence
- Strong hackmanite
- Pleochroism
- None
- Colours
- violet blue, blue, grey-blue
Formation, geology & history
A mineral of alkaline rocks
Sodalite crystallises in syenites and alkaline pegmatites, rocks poor in silica and rich in sodium. There it forms compact masses rather than fine crystals, generally associated with nepheline, calcite and white feldspars — hence the characteristic veining. Its density, between 2.1 and 2.4, is markedly lower than that of lapis: a piece of sodalite feels light in the hand, which is the quickest test to separate them. The Bancroft deposit in Canada was popularised in 1901 when the Princess of Wales had several tonnes shipped to decorate Marlborough House — hence the trade name «Princess Blue».
Hackmanite and tenebrescence
Some sodalites from Greenland, Afghanistan and Quebec possess a rare property: tenebrescence. Exposed to ultraviolet light they turn an intense violet pink; left in daylight they return to white or grey within minutes to hours. The cycle is reversible and can be repeated indefinitely. The phenomenon results from colour centres linked to sulphur ions in the lattice, switched on and off by light. These same hackmanites also fluoresce bright orange under short-wave UV, making them spectacular mineralogical display pieces.
Ornamental use
Quarried in large blocks, sodalite is mainly a decorative stone: trays, table tops, spheres, obelisks, beads and cabochons. Its very low price makes it ubiquitous in mineral shops. In jewellery it appears as cabochons or beads, but its modest hardness and cleavage limit it to pieces with little exposure. The main commercial issue remains confusion, deliberate or not, with lapis lazuli: watch for the absence of pyrite and for the lightness.
How to recognise it
- Light in the handDensity only 2.1-2.4: clearly lighter than lapis (2.7-2.9) for the same volume. This is the most reliable and quickest test.
- White veins without pyriteSodalite is marbled with calcite and white feldspar but never contains golden pyrite flakes.
- Violet tingeIts blue leans towards violet or royal blue, never towards the deep velvety ultramarine of Afghan lapis.
Imitations, treatments & confusions
- Sold as lapis lazuliThe most frequent deceptive use. Look for pyrite (absent) and weigh the piece (light): the diagnosis is immediate.
- Blue-dyed howliteEven softer, with fine cobweb-like black veins and a dye that bleeds onto damp cotton.
- Marbled blue glassBubbles, mould seams, swirled patterns. Warms quickly in the hand.
Deposits & origins
Brazil (Bahia — most of the ornamental market), Canada (Bancroft, Ontario — «Princess Blue» sodalite), Namibia, Greenland (fluorescent hackmanite from Ilímaussaq), Russia (Kola), Afghanistan, Bolivia
Care & precautions
- Brief rinsing in warm soapy water, dried at once. No soaking: the cleavage and slight porosity dislike prolonged moisture.
- No salt and no acids, which attack the mineral and the associated calcite.
- The colour may fade slightly under intense, prolonged sunlight.
- No ultrasonic cleaning: distinct cleavage makes the stone sensitive to vibration.
Traditions & crystal healing
- Chakras
- Throat · third eye
- Zodiac signs
- Sagittarius · Virgo
- Element
- Air
Tradition makes it a stone of mental clarity and honesty: it is said to help organise thought, speak accurately and reduce inner confusion. It is often recommended to students and people in training, placed on the desk rather than worn.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
Sodalite or lapis lazuli?
Three differences: sodalite is clearly lighter, more violet, and never contains golden pyrite. Its price is five to ten times lower.
What is hackmanite?
A sodalite that changes colour under UV (tenebrescence): it turns violet pink, then decolourises in daylight. The best specimens come from Greenland and Afghanistan.
Can it go in water?
A brief rinse, yes. Soaking, no: sodalite has distinct cleavage and keeps company with soluble calcite, which makes prolonged moisture harmful.