Is thomsonite an agate?
Is thomsonite an agate? No: a Na-Ca zeolite, hardness 5–5.5, density 2.23–2.39; agate is chalcedony, quartz, SG 2.60–2.64. On the balance a 0.68 g gap at 20 g.
No. Thomsonite is a zeolite species, thomsonite-Ca NaCa2(Al5Si5)O20·6H2O, hardness 5–5.5, density 2.23–2.39. Agate is a variety of chalcedony, which is quartz, SiO2, hardness 6.5–7, SG 2.60–2.64 on the International Gem Society’s figures. A 20.00 g thomsonite reads at most 11.63 g in water and an agate at least 12.31 g.
In short
- Two different things. On the September 2026 IMA-CNMNC Master List, thomsonite-Ca is NaCa2(Al5Si5)O20·6H2O and quartz is SiO2; by script, no line of the list contains agate or chalcedony. The International Gem Society agate page defines agate as a variety of chalcedony, a cryptocrystalline form of quartz. Thomsonite is a sodium calcium aluminosilicate zeolite with its own Handbook of Mineralogy sheet. It is not agate and not a form of quartz.
- The 0.68 g gap. Worked as 20 − 20/D, a 20.00 g piece reads 11.03–11.63 g in water as thomsonite-Ca at the sheet’s 2.23–2.39, and 12.31–12.42 g as agate at IGS’s 2.60–2.64. The gap is 0.68 g. On the least favourable pairing of thomsonite and agate figures, IGS’s thomsonite 2.40 against the quartz sheet’s massive 2.59, it is still 0.61 g. Weighing is the separation a collector can make on a loose, clean piece (how to measure specific gravity at home).
- The silica check. Worked from the IMA formula, thomsonite-Ca is 37.25% SiO2 and 31.60% Al2O3, with 13.91% CaO, 3.84% Na2O and 13.40% H2O; the sheet’s analyses print 37.17 and 41.49% SiO2. Agate’s formula, on the IGS panel and the quartz sheet, is SiO2. Under two-fifths of thomsonite is silica; all of agate is.
- Documented failures. The index does not separate them: thomsonite-Ca’s α 1.497–1.530 to γ 1.518–1.544 contains agate’s whole 1.530–1.543. The pattern does not either: the thomsonite sheet says its colours may be concentrically zoned, and IGS says agate patterns usually take the form of flat or concentric layers or bands. What the index can do, on a polished flat, is show birefringence: 0.014–0.021 worked for thomsonite-Ca against up to 0.004 for agate.
- When the answer would change. Only if the piece is not thomsonite. The sheet lists quartz among thomsonite’s associates, so a single piece can carry both, and a whole-piece weighing then reads a mixture. A label calling one stone thomsonite agate names two materials; which one is present is a question for the balance and, failing that, an analysis. Nothing on this site is offered for sale; the wanted list is the only commercial route.
| Material | What it is (source) | Formula | SiO2 by mass | Hardness | Density | 20.00 g in water | Refractive index | Birefringence | Pattern wording (source) |
|---|---|---|---|---|---|---|---|---|---|
| Thomsonite-Ca | Zeolite species; IMA row (sheet) | NaCa2(Al5Si5)O20·6H2O | 37.25% (analyses 37.17–41.49) | 5–5.5 | 2.23–2.39 | 11.03–11.63 g | α 1.497–1.530, γ 1.518–1.544 | 0.014–0.021 | May be concentrically zoned (sheet) |
| Thomsonite (IGS) | Same mineral, gem-society panel | NaCa2Al5Si5O20·6H2O | 37.25% | 5–5.5 | 2.25–2.40 | 11.11–11.67 g | 1.497–1.544 | 0.021 | Eyelike pattern (IGS) |
| Agate | Variety of chalcedony; no IMA row (IGS) | SiO2 | 100% | 6.5–7 | 2.60–2.64 | 12.31–12.42 g | 1.530–1.543 | Up to 0.004 | Flat or concentric layers or bands (IGS) |
| Chalcedony | Cryptocrystalline quartz; no IMA row (IGS) | SiO2 | 100% | 7 | 2.55–2.91 | 12.16–13.13 g | 1.540–1.553 | 0.004–0.009 | Not stated |
| Quartz | IMA species (sheet) | SiO2 | 100% | 7 | 2.65; 2.59–2.63 massive | 12.45 g; 12.28–12.40 g massive | ω 1.544, ε 1.553 | 0.009 | Zoned or mottled colour (sheet) |
Is thomsonite an agate? What each name means
No: thomsonite is a mineral species, and agate is a pattern variety of a different mineral, quartz. The International Gem Society agate page calls agate a remarkable variety of chalcedony, a cryptocrystalline form of quartz, and says its patterns, translucency and sometimes moss-like inclusions distinguish agates from other chalcedonies. Its property panel gives SiO2, hexagonal (trigonal) and cryptocrystalline, hardness 6.5–7, specific gravity 2.60–2.64, refractive index 1.530–1.543 and birefringence up to 0.004, with no cleavage. Those are the figures this site’s is carnelian an agate page uses.
Thomsonite, as recorded. The September 2026 IMA-CNMNC Master List has thomsonite-Ca, NaCa2(Al5Si5)O20·6H2O, and thomsonite-Sr, NaSr2(Al5Si5)O20·6-7H2O; the Sr member is recorded only in tiny crystals and zones from the Kola Peninsula, so a collector’s thomsonite is almost always thomsonite-Ca. The Handbook thomsonite-Ca sheet prints orthorhombic crystals, radiated spherical aggregates and smooth, globular, botryoidal, compact material; perfect {010} cleavage; hardness 5–5.5; density 2.23–2.39; and colours that may be concentrically zoned. It is a zeolite, as is thomsonite a zeolite sets out; the species page is thomsonite.
How do you tell thomsonite from agate? The 0.68 g gap
Weigh it in water: no thomsonite figure comes within 0.6 g of any agate figure for a 20.00 g piece. At the thomsonite-Ca sheet’s 2.23–2.39 a 20.00 g piece reads 11.03–11.63 g in water. At IGS’s agate 2.60–2.64 it reads 12.31–12.42 g, a gap of 0.68 g. Taking the least favourable pair of thomsonite and agate figures on this page, IGS’s thomsonite ceiling of 2.40 (11.67 g) against the quartz sheet’s massive floor of 2.59 (12.28 g), the gap is 0.61 g. The method is on how to measure specific gravity at home.
Hardness agrees. Thomsonite is 5–5.5 on its sheet and on the IGS panel; agate is 6.5–7 (IGS) and quartz 7 (sheet), so agate scratches thomsonite and not the reverse; the method is on how to test mineral hardness properly. Thomsonite’s perfect {010} cleavage is a third clue on a broken piece, since agate has none.
Documented narrowing: chalcedony’s floor. The International Gem Society chalcedony page gives chalcedony in general a specific gravity of 2.55–2.91, wider than agate’s. At 2.55 a 20.00 g piece reads 12.16 g, still 0.49 g above IGS’s heaviest thomsonite. The gap narrows against the broadest chalcedony figure but does not close.
Why does thomsonite look like agate? Two failed tests
Both can be concentrically patterned, and their refractive indices overlap. The thomsonite-Ca sheet says its colours may be concentrically zoned, and the International Gem Society thomsonite page describes a pinkish gray eyelike pattern in some thomsonite cabochons. IGS says agate’s colour patterns usually take the form of flat or concentric layers or bands. A zoned, rounded thomsonite and a banded agate can therefore look alike, and pattern is the one property the two names share.
Documented failure: the refractometer. Thomsonite-Ca’s indices run from α 1.497–1.530 to γ 1.518–1.544, and IGS puts thomsonite’s shadow edge at 1.52–1.54. Agate reads 1.530–1.543. The thomsonite range contains agate’s whole range, so a single spot reading does not decide. On a polished flat the birefringence can help: worked from the sheet, thomsonite-Ca’s γ − α is 0.021 at the low ends of its ranges and 0.014 at the high ends, and IGS prints 0.021; agate’s is up to 0.004.
Not the boundary that matters. Colour and lustre do not separate them either: the thomsonite sheet lists white, yellowish, pink, brown and greenish, vitreous to pearly; IGS gives agate all colours, greasy to waxy. Whether a banded quartz is agate, onyx or carnelian is a separate question, answered on is carnelian an agate, is onyx a mineral and is chalcedony quartz.
When would a thomsonite be an agate?
Never as the same material; only as two materials on one piece. The thomsonite-Ca sheet lists zeolites, calcite, prehnite, datolite and quartz as associates, so a single specimen can carry thomsonite and quartz together. A whole-piece weighing then reads a mixture, between the two lines, and proves nothing either way. Neither the thomsonite sheet nor the IGS thomsonite page describes thomsonite replaced by chalcedony, and this page does not suggest it happens.
Reading a label. A label calling one stone both thomsonite and agate names two materials. If the piece weighs on the thomsonite side of the gap and is softer than quartz, thomsonite is the defensible word; if it weighs on the agate side and shows no cleavage, chalcedony or agate is. Anything finer needs an analysis, the basis of every name on thomsonite. More guides are in the collecting guides.
Is thomsonite toxic? Whether a zeolite specimen is a hazard is a question this page does not answer; it is routed to toxic minerals in a collection, and nothing here is advice. Nothing on this site is offered for sale; the wanted list is the only commercial route.