What is Prehnite?
Prehnite is an inosilicate of calcium and aluminium. Prehnite crystallises in the orthorhombic crystal system, and most often forms as stalactitic or botryoidal aggregates, with only just the crests of small crystals showing any faces, which are almost always curved or composite. Very rarely will it form distinct, well-individualised crystals showing a square-like cross-section, including those found at the Jeffrey Mine in Asbestos, Quebec, Canada. Prehnite is brittle with an uneven fracture and a vitreous to pearly luster. Its hardness is 6-6.5, its specific gravity is 2.80-2.90 and its colour varies from light green to yellow, but also colourless, blue, pink or white. In April 2000, rare orange prehnite was discovered in the Kalahari Manganese Fields, South Africa. Prehnite is mostly translucent, and rarely transparent.
Though not a zeolite, prehnite is found associated with minerals such as datolite, calcite, apophyllite, stilbite, laumontite, heulandite etc. in veins and cavities of basaltic rocks, sometimes in granites, syenites, or gneisses. It is an indicator mineral of the prehnite-pumpellyite metamorphic facies.
History and Introduction
Prehnite is a translucent to transparent gem-quality hydrated calcium aluminum silicate. It was the first mineral to be named after an individual, and it was also the first mineral to be described from South Africa, long before South Africa became one of the most important sources for precious and semi-precious gems. It was first described in 1788 after it was discovered in the Karoo dolerites of Cradock, South Africa. Prehnite was later named after its discoverer, Colonel Hendrik von Prehn (1733-1785), a Dutch mineralogist and an early governor of the Cape of Good Hope colony.
Until recently, prehnite was a rare collector's gemstone, but new deposits have now made it more readily available. In China, prehnite is sometimes referred to as 'grape jade' owing to its typical nodule formations which often resemble a bunch of grapes. Its colour is usually a soft apple-green, which is quite unique to prehnite, but it can also occur in rarer colours including yellow, orange and blue.
Identifying Prehnite
Prehnite is typically semi-transparent to translucent with a chemical formula of Ca2Al(AlSi3O10)(OH)2. Its color is usually yellow-green to apple-green. Prehnite is considerably hard with a rating of 6 to 6.5 on the Mohs scale. It has a specific gravity ranging from 2.82 to 2.94 and a refractive index of 1.611 to 1.669. Prehnite is in the orthorhombic crystal class, usually found in radiating botryoidal (grape-like) aggregate forms, and rarely as tabular and pyramidal crystals. When heated, prehnite crystals can sometimes give off water. It has a brittle tenacity and an uneven fracture. When polished, prehnite has a vitreous to pearly luster. Prehnite may be confused with apatite, jade or serpentine.
Prehnite: Origin and Sources
Prehnite occurs in the veins and cavities of mafic volcanic rock. It is a typical product of low-grade metamorphism. Primary deposits of prehnite are sourced from several locations around the world. Some of the most important deposits come from Africa (Namibia, South Africa), Australia (Western Australia, Northern Territory), Canada, China, Germany, Scotland, France and the United States (New Jersey, Pennsylvania and Virginia).
Rare, orange coloured prehnite has been discovered in South Africa. Quebec, Canada is known to produce prehnite with distinct, individual crystals.
Prehnite: Related or Similar Gemstones
There are no closely related gemstones, but there are several gemstones which can have a very similar appearance (colour and luster), including jade, apatite, serpentine, brazilianite, periclase, chrysoprase, peridot, smithsonite and hemimorphite. Prehnite is also often found and associated with many microporous, aluminosilicate zeolite minerals such as datolite, calcite, apophyllite, stilbite and heulandite.
Prehnite Healing properties
Prehnite is considered a stone of unconditional love and the crystal to heal the healer. It enhances precognition and inner knowing. Enables you always to be prepared. Prehnite calms the environment and brings peace and protection. It teaches how to be in harmony with nature and the elemental forces. Helpful for “decluttering” letting go of possessions you no longer need, aiding those who hoard possessions, or love, because of an inner lack. Prehnite alleviates nightmares, phobias and deep fears, uncovering and healing the dis-ease that creates them. It is a stone for dreaming and remembering. Beneficial for hyperactive children and the causes that underlie the condition.
Prehnite heals the kidneys and bladder, thymus gland, shoulders, chest and lungs. It treats gout and blood disorders. Prehnite repairs the connective tissue in the body and can stabilise malignancy.
Properties of Prehnite
| Chemical Formula | Ca2Al2Si3O12(OH) |
| Colour | Green |
| Hardness | 6 - 6.5 |
| Crystal System | Orthorhombic |
| Refractive Index | 1.61 - 1.64 |
| SG | 2.8 - 3.0 |
| Transparency | Translucent |
| Double Refraction | .030 |
| Luster | Vitreous, waxy |
| Cleavage | 1,1;3,1 |
| Mineral Class | Prehnite |
