Gem library
Celestite
Sky blue from strontium sulphate
Celestite is the world's main source of strontium, and incidentally one of the loveliest blue geodes nature makes. Its fragility confines it to a shelf: hardness 3, perfect cleavage, and a colour that fades in daylight.
- Hardness (Mohs)
- 3 – 3.5
- Density
- 3.96 – 3.98
- Crystal system
- Orthorhombic
- Refractive index
- 1.622 – 1.631
- Cleavage
- Perfect on {001}
Mineralogical identity
- Family
- Sulphates
- Species
- Celestine
- Variety
- Blue celestite
- Chemical formula
- SrSO₄
- Crystal system
- Orthorhombic
- Classification
- Collector's mineral
- Rarity
- Uncommon
- Indicative price
- €0.50 – 15/ct
Etymology
From the Latin caelestis, «heavenly», for its pale sky colour. The name was given in 1799 by the German mineralogist Abraham Werner.
Physical & optical properties
- Hardness (Mohs)
- 3 – 3.5
- Density
- 3.96 – 3.98
- Refractive index
- 1.622 – 1.631
- Birefringence
- 0.009
- Cleavage
- Perfect on {001}, distinct on {210}
- Fracture
- Conchoidal
- Lustre
- Vitreous to pearly
- Diaphaneity
- Transparent to translucent
- UV fluorescence
- Weak
- Pleochroism
- Weak
- Colours
- sky blue, pale blue, colourless, white
Formation, geology & history
Formation and geology
Celestite is deposited in sedimentary rocks, above all in limestones and dolomites where strontium-rich brines circulate, and in evaporites and around some hydrothermal springs. It lines cavities where crystals grow freely, which is what produces the geodes carpeted in sky-blue prisms that make its reputation. The Sakoany geodes in Madagascar formed in an Eocene limestone; these are what you see in almost every shop. Those of Put-in-Bay, in Ohio, reach spectacular size: the largest known cavity is more than ten metres across, entirely lined with crystals, some over forty centimetres long.
Strontium, and what it is for
Celestite is the strontium ore par excellence. For a century that strontium went into the glass of cathode-ray screens, where it blocked the X-rays the tube emitted: the disappearance of those screens collapsed demand. What remains is pyrotechnics, where strontium salts give fireworks their vivid red, aluminium metallurgy, and certain permanent magnets. Strontium nitrate is still the reference red pigment for distress flares. A close relative, barite, barium sulphate, often occurs in the same deposits and is hard to tell apart by eye.
Colour and fragility
The pale blue comes from defects in the crystal structure rather than from a colouring element, which explains how fragile it is in light: a geode left in daylight loses its colour in months, sometimes weeks behind a south-facing window. Collectors keep their pieces in the dark and bring them out only to look at them. Perfect cleavage combined with hardness 3 means a crystal breaks if you squeeze it between two fingers. The rare faceted celestites are display pieces, never stones to wear.
How to recognise it
- A density that surprises the handClose to 4.0: for its size, a piece of celestite weighs noticeably more than a quartz or a calcite of the same bulk. Hold both and the difference is obvious.
- Hardness 3A fingernail will not scratch it, a copper coin barely does, a steel point does so easily. A blue stone that resists steel is not celestite.
- The red flameA chip heated in a flame colours it carmine red: the signature of strontium. A destructive test, for a worthless fragment only.
Imitations, treatments & confusions
- Dyed geodesQuartz or agate geodes dyed blue are sold as celestite. Quartz scratches glass, celestite does not: the test settles it in a second.
- Confusion with bariteSame habit, same deposits, but barite is denser still (4.5) and rarely this blue. The reliable distinction is the flame test, yellow-green for barium.
- Angelite sold as celestiteAngelite is anhydrite, an opaque, matt calcium sulphate. Celestite is translucent and crystallised. The price is not the same.
Deposits & origins
Madagascar (Sakoany, the blue geodes of the trade), United States (Ohio, Put-in-Bay), Mexico, Germany, United Kingdom (Bristol, the historic deposit), Poland, Egypt
Care & precautions
- Never water: the cleavage and low hardness mean washing detaches chips. A soft, dry brush is enough.
- Away from daylight, always. The blue fades and does not come back.
- Never lay a geode on its side: its own weight is enough to snap the crystal tips.
- No ultrasonics, no steam, no household products.
Traditions & crystal healing
- Chakras
- Throat · third eye · crown
- Zodiac signs
- Gemini · Libra
- Element
- Air
It is linked to gentleness and calm speech, and is more readily placed in a bedroom than in a workspace. Its reputation owes as much to its sky colour as to its name. Cultural heritage, never medicine.
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
Why has my celestite turned white?
It has seen daylight. The blue comes from structural defects that light erases, and the process is irreversible. Keep geodes in a closed cabinet.
Can celestite be set in jewellery?
Technically yes, sensibly no. Hardness 3 and perfect cleavage: the stone wears and splits within weeks. It is a cabinet mineral.
Celestite or celestine?
Both are current. «Celestine» is the official mineralogical name, «celestite» the form the trade prefers. Same mineral.
Tools for this stone
Celestite: only the tools this stone actually concerns, each carrying the value read on this page.