Is Prehnite UV Reactive?
Is prehnite UV reactive? Its Handbook sheet prints no fluorescence. Twelve database records show blue, orange, pink, violet, yellow or green, by locality.
Sometimes. The Handbook of Mineralogy prehnite sheet prints no fluorescence at all, but the Fluorescent Mineral Society’s database links 12 prehnite records with responses: 7 under longwave UV (6 blue, 1 orange) and 6 under shortwave (violet, pink, yellow, green). They come from only 5 localities, and the colour changes from place to place and even within Franklin.
In short
- The sheet is silent. A ligature-aware search of the Handbook of Mineralogy prehnite sheet for fluoresc-, luminesc-, phosphor- and UV finds nothing, while the same search finds the printed wording on the hemimorphite and willemite sheets. Silence is not a test result: it means the sheet does not record a response, not that none exists.
- The 12-record ledger. The Fluorescent Mineral Society’s database (FMDB) links 12 specimen records to prehnite. Read one by one, they give 7 longwave responses (blue in 6, orange in 1), 6 shortwave (violet in 3, and pink, yellow and green), and 1 midwave (green). Only one record, #181 from Merelani, Tanzania, lists more than one lamp: blue longwave, green midwave and green shortwave.
- Documented failure: 12 records are not 12 tests. Five are Franklin, New Jersey, three are one Quebec find posted three times, and two are Merelani: 5 localities in all. Two Franklin records hedge the identification (“presumably, prehnite”; “I think mixed together”), and record #461 is described as from the Jeffrey mine, Asbestos, but its locality field names a different Jeffery mine in Ontario.
- Two more sources, two more answers. The Fluomin prehnite record lists longwave yellowish white and orange and shortwave from bluish white to orange, with Franklin and the Jeffrey mine among its best localities. The International Gem Society prehnite page says only “may be dull brownish yellow in UV and X-rays”. The three describe the response differently, which is what a locality-dependent response looks like.
- Look at the neighbours. Prehnite’s sheet names zeolites, datolite, pectolite, calcite, epidote and copper as associates. The calcite sheet says calcite may fluoresce many colours under SW and LW UV, and the datolite sheet that datolite commonly fluoresces blue under SW UV. A glow on a prehnite specimen has to be located on the prehnite before it is credited to it.
| FMDB record | Posted | Locality field (first part) | Longwave | Midwave | Shortwave | Identification as stated |
|---|---|---|---|---|---|---|
| #1 (with roeblingite) | Dec 2024 | Franklin Mine, New Jersey | — | — | Violet | “grains of, presumably, prehnite” |
| #29 | Dec 2024 | Imilchil, Morocco | Blue | — | — | Rosette of light green prehnite |
| #181 | Jul 2025 | Merelani Hills, Tanzania | Blue | Green | Green | Yellow prehnite; LW peak at 449 nm |
| #461 | Mar 2026 | Jeffery Mine, Parry Sound District, Ontario | Orange | — | — | Described as Jeffrey Mine, Asbestos |
| #477 | Mar 2026 | Franklin Mine, New Jersey | — | — | Pink | Prehnite |
| #929 | Apr 2026 | Merelani Hills, Tanzania | Blue | — | — | Prehnite; best response under LW |
| #1073 (with margarosanite) | Apr 2026 | Franklin Mine, New Jersey | — | — | Yellow | “tan-fluorescing prehnite” |
| #1391, #1392, #1393 | May 2026 | Grenville-sur-la-Rouge, Québec | Blue (each) | — | — | Prehnite by Raman, with scapolite and calcite; one find |
| #1632 (Franklin mix) | Sep 2026 | Franklin Mine, New Jersey | — | — | Violet | Prehnite among several species |
| #1634 (with nasonite) | Sep 2026 | Franklin Mine, New Jersey | — | — | Violet | “Mostly nasonite and prehnite, I think mixed together” |
| Totals, 12 records | — | 5 localities | 7: blue 6, orange 1 | 1: green | 6: violet 3, pink, yellow, green | — |
Does prehnite glow under UV? What the Handbook sheet says
Nothing: the prehnite sheet prints no fluorescence, phosphorescence or UV response. Older Handbook sheets render the “fl” ligature as a stray glyph, so the search looks for the stem “uoresc” after any character, plus luminesc-, phosphor-, “UV” and “ultraviolet”, case-insensitively. It finds “may °uoresce bluish under SW UV” on the hemimorphite sheet and “strongly °uorescent … under SW and LW UV” on willemite’s, the two positive controls, and nothing on prehnite’s. The quartz sheet prints only “may be triboluminescent”, which is light from rubbing, not UV.
The species itself. The prehnite sheet gives Ca2Al2Si3O10(OH)2, which the September 2026 IMA-CNMNC Master List writes Ca2Al(Si3Al)O10(OH)2 (G, 1788, South Africa): orthorhombic, hardness 6–6.5, density 2.80–2.95, light to dark green, white, yellow, gray or pink, with a vitreous lustre and a white streak. Why it is not a zeolite, despite its company, is on is prehnite a zeolite.
What silence means. A sheet that records nothing has not recorded a negative test. The answer therefore has to come from collections where someone put prehnite under a lamp and wrote down what happened, and the next section reads one such collection record by record.
What colour does prehnite fluoresce? The 12-record ledger
Blue under longwave is the commonest record, but it is one colour among six. The Fluorescent Mineral Society’s database (FMDB) page for prehnite links 12 specimen records. Their summaries list a prehnite response in each: under longwave (365 nm), blue in FMDB #29, FMDB #181, FMDB #929 and the three Grenville-sur-la-Rouge records, and orange in FMDB #461; under midwave, green in #181; under shortwave, violet in FMDB #1, FMDB #1632 and FMDB #1634, pink in FMDB #477, yellow in FMDB #1073 and green in #181. The full ledger is the table above.
What the records add. #181’s contributor measured the Merelani longwave emission as a broad peak with a maximum at 449 nm and wrote that there is no available information on what activates it; this page does not offer an activator either. The Grenville contributor had the blue-glowing grains identified as prehnite by Raman spectroscopy, alongside yellow-fluorescing scapolite (is scapolite fluorescent). #929’s contributor found the best Merelani response under longwave.
Documented failure: 12 records, 5 localities. Five records are from the Franklin mine (Franklin and Sterling Hill), three are the same Quebec find posted on the same day, and two are Merelani. Two Franklin records do not stand as prehnite tests on their own: #1 calls the violet grains “presumably, prehnite, … possibly mixed with other minerals”, and #1634 describes “mostly nasonite and prehnite, I think mixed together”, with Raman showing both minerals in both phases. And #461 is described as from the Jeffrey mine at Asbestos, the Quebec locality both the prehnite sheet and Fluomin name, while its locality field names a Jeffery mine in Parry Sound District, Ontario; this page reports the orange response and leaves the place unresolved.
Do other sources agree? Fluomin and the gem reference
They agree that prehnite can respond, and on little else. The Fluomin prehnite record lists longwave colours yellowish white and orange, midwave pale yellow, and shortwave bluish white, white, yellowish white, pale yellow and orange, with no visible phosphorescence data. It also records bluish fluorescence under longwave at the Jeffrey mine plus an orange-fluorescing coating by an unknown mineral, which matters for #461’s orange. Its best localities include the Franklin mine and the Jeffrey mine. It names an activator; this page does not repeat activator claims.
The gem reference. The International Gem Society prehnite page gives “May be dull brownish yellow in UV and X-rays” and marks the luminescence type as fluorescent, UV-long. Set beside the FMDB’s blue and Fluomin’s yellowish white, it is a third description, not a confirmation of either.
A rule a collector can use. Read prehnite’s response as a property of the piece and its locality, not of the species: write the lamp, the colour and the place on the label, and expect a piece from another locality to differ or not respond at all. The same approach is used on is charoite UV reactive and is wavellite fluorescent, where a sheet is silent and the records disagree.
Is the glow from the prehnite? Check the associates first
Prehnite grows with species whose sheets do print fluorescence, so a glowing spot must be traced to the prehnite before it counts. The prehnite sheet’s association line is zeolites, datolite, pectolite, calcite, epidote and copper. Run through the same ligature-aware search, the calcite sheet says calcite may fluoresce red, blue, yellow and other colours under either SW and LW UV; the datolite sheet says it commonly fluoresces blue under SW UV (datolite); pectolite’s prints only “may be triboluminescent” (pectolite); and the epidote and copper sheets print nothing.
In practice. Four of the 12 FMDB records are multi-species Franklin pieces, #1391 from the Grenville find also shows greenish fluorescing, phosphorescent calcite, and #181 reports a calcite crystal fluorescing orange red beside the Merelani prehnite. Calcite’s own behaviour is on is calcite UV reactive, and the general picture is on fluorescent minerals explained. More fluorescence pages are in the collecting guides.
Lamp choice is covered on best UV lamp for mineral collecting. Exposure limits for ultraviolet radiation from 180 to 400 nm are in the ICNIRP 2004 guidelines on limits of exposure to ultraviolet radiation; this page is not safety advice, and other hazards are routed to toxic minerals in a collection. Nothing on this site is offered for sale; the wanted list is the only commercial route.