Is celestite a quartz
No. Celestite, IMA name celestine, is strontium sulphate, hardness 3 to 3.5, density 3.97; quartz is silica at 7 and 2.65. Steel or the 3.31 line decides.
No. Is celestite a type of quartz? Celestite, the IMA's celestine, is strontium sulphate, SrSO4, hardness 3 to 3.5, measured density 3.97; quartz is silicon dioxide, SiO2, hardness 7, density 2.65. Both can be pale blue, which is where the confusion starts. A steel point marks celestine and not quartz.
In short
- Different compounds. Celestine is strontium sulphate, 47.70% strontium by mass, in the barite group; quartz is silicon dioxide. The IMA name is celestine, which the celestine species page uses and this page uses from here on.
- The 3.5-point rule. Celestine is hardness 3 to 3.5 and quartz 7. A steel point that bites is consistent with celestine; one that skates is consistent with quartz, which also scratches glass.
- The 3.31 line. Celestine's measured density is 3.97 and quartz's 2.65. A 20.00 g piece weighs 14.96 g suspended in water as celestine and 12.45 g as quartz, 2.51 g apart. Below 3.31, quartz; above it, not quartz.
- Cleavage agrees. Celestine cleaves perfectly on {001} and well on {210}, and is brittle; the quartz sheet says its cleavage is rarely observable, with a conchoidal fracture.
- A blue mineral that resists steel is unusual. Of the 176 sheets in this site's reference set, 54 list blue in the Color field and 6 of those reach hardness 7 or more, quartz among them. Celestine's sheet is not one of the 176.
| Criterion | Celestine | Quartz | Does it choose? |
|---|---|---|---|
| Formula | SrSO4 | SiO2 | Yes, but needs a laboratory |
| Strontium by mass | 47.70% ideal; 47.52% (Djebel Kebbouch analysis) | None | Only by analysis |
| Hardness (Mohs) | 3 to 3.5 | 7 | Yes: 3.5 points apart |
| Density, measured | 3.97 | 2.65; 2.59 to 2.63 when massive | Yes: cut-off 3.31 |
| 20.00 g piece, weight in water | 14.96 g | 12.45 g; 12.28 to 12.40 g massive | Yes, even on a 0.1 g scale |
| Cleavage | {001} perfect; {210} good; {010} poor | Rarely observable; conchoidal fracture | Yes, on a broken edge |
| Crystal system; largest crystals | Orthorhombic; to 45 cm | Hexagonal; to 6 m | Yes, when crystals are present |
| Colour as recorded | Colorless, white, pale blue, pink, pale green, pale brown, black | Colorless, white; blue among the colours from inclusions | No: both can be pale blue |
| Lustre | Vitreous, pearly on cleavages | Vitreous; waxy to dull when massive | Weakly |
| IMA status (Sept 2026) | Approved, 1967 s.p.; USA | Approved, 1967 s.p. | - |
Is celestite a type of quartz? Strontium sulphate against silicon dioxide
Celestite is not quartz: it is strontium sulphate, and quartz is silicon dioxide. The Handbook of Mineralogy prints celestine as SrSO4, in the Barite group, forming a series with barite, and quartz as SiO2. The IMA-CNMNC list of minerals lists the species as celestine, approved with the year 1967 s.p. and the USA as the country of first description; celestite is the same mineral under its common name, and the sheet's own reference list uses both. The name is from the Latin for celestial, for the typical bluish colour.
Strontium, worked. With Sr 87.62, S 32.06 and O 15.999, SrSO4 has a formula mass of 183.68 and is 47.70% strontium by mass. The sheet's pure column gives SrO 56.41%, which converts to the same 47.70%, and its Djebel Kebbouch, Tunisia, analysis has 56.20% SrO, 47.52% strontium. The Handbook calls celestine the most common strontium mineral.
The celestine sheet is not one of the 176 in this site's reference set; it was fetched as a comparator and is counted in no survey figure on this page. Two of the 176 are in the Barite group: baryte and anglesite.
The 3.5-point rule: why a steel point decides
Celestine and quartz are 3.5 Mohs points apart. Celestine is hardness 3 to 3.5 and quartz 7. A steel point, conventionally about 5.5, marks celestine and skates on quartz; quartz scratches window glass and celestine does not. Test on the base or matrix, wipe the mark, and look again, because a soft point can leave a metal streak that reads as a scratch; the method is on how to test mineral hardness properly.
Cleavage says the same thing on a broken edge. Celestine cleaves perfectly on {001}, well on {210} and poorly on {010}, and its lustre turns pearly on cleavages. Quartz's sheet records cleavage as rarely observable and its fracture as conchoidal. Flat, stepped, pearly breaks point to celestine; curved, glassy shell-like breaks point to quartz. Reading a break is covered on cleavage or fracture.
The 3.31 line: a worked density test on a 20 g piece
On a 20.00 g specimen, celestine and quartz are 2.51 g apart in water. At celestine's 3.97 a 20.00 g piece has a volume of 5.04 cm³ and weighs 14.96 g suspended in water. At quartz's 2.65 the volume is 7.55 cm³ and the water weight 12.45 g; massive quartz, at 2.59 to 2.63, gives 12.28 to 12.40 g. The midpoint of 2.65 and 3.97 is 3.31. The method and the scale resolution it needs are on how to measure specific gravity at home.
Where it fails. A crust of celestine on rock, or blue crystals on a quartz-lined cavity wall, weighs as a mixture. The hardness test on a single crystal is the check. A closer confusion for celestine is its series partner baryte, at 4.50: a 20.00 g piece of baryte weighs 15.56 g in water, about 0.6 g more than celestine, and the two are separated by a cut-off near 4.24, not by hardness, since both are 3 to 3.5.
How many blue minerals resist a steel point? 6 of 54 sheets
Of the 176 Handbook sheets in this site's reference set, 54 list blue in the Color field, and 6 of those reach a printed hardness of 7 or more: corundum, forsterite, quartz, topaz, vesuvianite and zoisite. Method: pdftotext -layout text, the Color field cut out by regular expression and searched case-insensitively for blue, then the first figures after Hardness = read by script. At the other end, 27 of the 54 have a printed hardness that tops out at 3.5 or below; celestine, from outside the set, would sit with them. A pale blue crystal that shrugs off steel is therefore a short list, and celestine is not on it.
Who the answer quartz is wrong for: anyone holding pale blue tabular crystals that a steel point marks and that feel heavy for their size. Who the answer celestine is wrong for: anyone whose blue piece scratches glass. Who neither suits: a heavy white sulphate that could be celestine or baryte, which density at the 4.24 line separates. Telling quartz from the other common look-alike is on how to tell calcite from quartz; the guides are indexed at the collecting guides. Nothing here is offered for sale; the wanted list is the only commercial route.