silicates
Prehnite
Ca2Al(Si3Al)O10(OH)2
- Crystal system
- orthorhombic
- Hardness
- 6–6.5 on the Mohs scale
- Lustre
- vitreous
- Colour
- Pale yellow-green to apple green
- Where it forms
- In cavities in basalt, with zeolites, and in some metamorphic rocks.
- Chiefly from
- Cape Province, South AfricaMaliAustraliaScotland
Why it is that colour
Iron. The characteristic soft translucent green with a faintly cloudy interior is distinctive.
Safety
Safe to handle.
Where it comes from
Malian and Australian material supplies most of the market. Inexpensive.
Historical use
Geologically useful: prehnite defines the prehnite-pumpellyite facies, a specific low-grade metamorphic zone.
Recorded as history and attributed where possible. That a stone was used for something establishes that people used it, not that it worked.
What is attributed to it, and since when
Described in 1788 and named after Colonel Hendrik von Prehn, who brought the first specimens from the Cape. It was the first mineral ever named after a person, and it has no traditional attribution.
Described, not endorsed. Dating a claim is not the same as assessing it — but an association that first appears in a book published in 1985 is not an ancient inheritance, whatever the label says. A sincere account of feeling better is compatible with the stone doing nothing — see the placebo effect.
Worth knowing
Prehnite has one small distinction: it was the first mineral to be named after a person, in 1788, and it set the convention that has since produced several thousand mineral names ending in -ite attached to somebody's surname.
It is worth registering how young mineral nomenclature is. Quartz, gold and sulfur have ancient names. The overwhelming majority of the six thousand approved species were named in the last two hundred years, by people, after people. A naming system that recent cannot be carrying much ancient meaning with it.
Identity checked against IMA-CNMNC master list, June 2025.