Semi-Precious Gemstones: Prehnite
Prehnite used in Jewelry
Source: Australia, Canada, China, Germany, India, Namibia, Scotland, South Africa, USA
Prehnite (French: Chrysolite du Cap, German: Adilit, Aedelit) is a relatively rare Calcium Aluminum Silicate Hydroxide mineral that was named after Dutch Colonel Hendrik von Prehn (1733-1785), when it was discovered in 1788.
Prehnite is a pinacoidal phyllosilicate (Calcium Aluminum Silicate Hydroxide) that typically occurs within cavities of vesicular mafic igneous rock that formed through hydro-thermal action. Prehnite typically occurs in association with aluminosilicate minerals (zeolites) such as datolite, ferrierite, quartz, calcite, pectolite, and copper minerals.
Prehnite is known in China as Putao Yu, or "Grape Jade" due to its nodular crystal habit which can appear like a bunch of pale-green grapes. Prehnite exhibits pseudochromatic coloration which give it a distinct pearlescent (mother-of-pearl) effect.
Prehnite is translucent or semi-transparent to opaque and is typically cut into cabochons, and is rarely facetted (see: SunJade® prehnite below). Prehnite could be mistaken for apetite, peridot, or jade.
Prehnite Crystallography, Chemistry, Physical Properties
Prehnite Optical Properties
Australian Prehnite and SunJade®
Article by Gerald Pauley
Australia is rich in mineral and gemstone resources but a coordinated mining operation to extract many of these gemstones has never been instigated in Australia. The main reason for this is the lack of developmental capital amongst the prospectors to explore for, peg claims, purchase gemstone tenements and also purchase suitable equipment for the extraction and shipping of the raw materials from the mine to the port for export. The wealth of Australian gemstones is untapped and the beautiful and rare gemstone, prehnite, because of its diversity of colour range, gem quality and cat's eye effect, could effectively be marketed as a true precious gem.
Prehnite Mining in Australia
"Prehnite is regarded as one of the world's rare precious gems..." from Minerals of The World by Rudolf Duda and Lubos Rejl.
Until recently, this statement could be considered to be correct until my discovery of the gem grade deposits in Northern Territory during the 1980's. Although prehnite is a relatively common mineral, especially in basalts, the gem variety of prehnite is considered rare. Because of the scarcity of high grade gem prehnite, Sun Jadeª, the material has not been marketed internationally. The deposits that we have, contained in three tenements totalling 200 square kilometres, are proving to contain a large quantity of the high gem grades suitable for high grade cabochons and faceted stones. Previously, prehnite was not used in jewellery because of it's insipid colour but with the discovery of yellow prehnite at our deposits prehnite should gain acclaim in the jewellery world. The surface material has been suncrazed. and faded due to exposure to the hot sun and extremes of temperature for hundreds of years.
Only inches below the surface, the colour improved and the material was of a far superior quality, becoming more transparent with depth. The colours range from pale green to dark green, and from pale yellow to an almost golden colour. On the international market, there are few natural yellow gemstones available in large quantities.
The current world gemstone market has yet to fully understand prehnite and how to cut the gem grade material. Our prehnite is a truly unusual stone. Prehnite cannot, and should not, be compared to the ornamental gemstones which are being mass produced in Asia. With a hardness of 6 to 6.5, prehnite is not a soft gem. It is also a tough stone that is suitable for high quality jewellery. Compared to Australian chrysoprase, a popular green gemstone in Asia, prehnite has many advantages. Chrysoprase is an opaque to semi-translucent stone, whereas prehnite is translucent to semi-transparent and can be faceted. Our high-quality gem grade prehnite could, therefore, fall into a similar category to chrysoprase and be highly prized in Asia as a "Jade-type" gemstone.
One unique characteristic of prehnite is its ability to "glow" in sunlight. The radial fibrous structure of the gem is also an advantage in that, if cut correctly, it can exhibit a catÕs eye, or "moonstone" (adularescence) effect.
Australian Prehnite Deposits
The prehnite deposit is situated at the northern edge of the Tanami Desert in Northern Territory. It is semi-arid plateau country with sparse vegetation, mainly used for cattle grazing. A variety of trees line the creeks but the open plains consist mainly of spinifex and Flinder's Grass.
In many places within our tenements, prehnite litters the surface, and with a soil depth of a few inches the host rock, basalt, is easily reached by manual digging. The surface material is usually well weathered and a true indication of the quality has been difficult to estimate. Surface material is faded and suncrazed but just a few inches below the surface, embedded in the basalt the quality of the material increases dramatically. From a small excavation, we retrieved over 60 Kgs of material, all of which was better colour and less sun-crazed than material on the surface. The transparency was also better than the surface material. Additionally, there was a higher percentage of yellow material and some nodules were a very dark green colour.
Australian Prehnite Valuation
The internet has brought the world closer and it is this advertising medium that has attracted buyers for prehnite. Graysons Enterprises, in their rare precious stone catalogue, quoted $US60.00 per carat for a 4.90 carat cut prehnite. From the description, it would not be as good as our top green coloured material and the yellow material is considerably better and could, therefore command much higher prices.
In 1988, an associate took an 8 carat bright yellow faceted prehnite to the USA and had the stone valued at $US200.00 per carat. On a recent trip for China we confirmed a valuation on our 'AA Grade' rough prehnite of $US100.00 per KG and $US50.00 per Kg. for 'A Grade' material. It was also confirmed that our yellow prehnite is the some of the best in the world and could command prices higher than $US300.00 per Kg for rough material.
Marketing of Australian Prehnite
Throughout history, marketing has played a very important part in the growth in popularity of many gemstones. With marketing, typically comes a trade name. A classic example of selling a gemstone under a trade name was witnessed when a company discovered a rare and beautiful gemstone mineral called sugilite. Two separate companies marketed the gem under the trade names of 'Royal Azel' and 'Royal Lavulite.' Both names were accepted and and this did not affect the sale of or demand for the gems. Sugilite was advertised as one of the newest and rarest gemstones, and although at the time it was certainly one of the newest, it was not all that rare as over 6,000 kilograms were mined. The price of the highest grades varied from $US600.00 to $US 1,000 per carat with lower grades selling as specimen gems. Rarity alone does not always determine whether a gem will command a high price. A stone must have colour, beauty, durability and rarity to possess the attributes of a true gem material. Sometimes, rarity can be a disadvantage in marketing the gem to achieve it's best potential if continuity of supply cannot be guaranteed.
With prehnite, we can guarantee a continuity of supply of high-quality gems and therefore, this makes prehnite a contender for a mass marketing strategy as a beautiful, and rare gemstone. Prehnite has a unique advantage in that it has a multitude of uses in the gemstone and gift industries. It can be carved into figurines, it can be faceted, and it has a unique feature of a moonstone and cat's eye effect when cut into cabochons.
It was once observed that prehnite "... looks just like bottled sunlight!" That was a very apt statement, and in keeping with this observation, we have chosen a marketing name symbolic of that particular characteristic of prehnite. The name is "Sun Jade."
At the dawn of the Cambrian Era, over 530 million years ago, many parts of Australia were engulfed by lava from active volcanoes. One area in the north witnessed the largest flow of volcanic lava recorded in Australia, covering an area of 250,000 Square Kilometres to a depth of up to 1,000 metres. The Sun Jade subsequently formed inside gas bubbles within the cooling lava.
The Antrim Plateau Volcanics is the largest flood basalt province in Australia and outcrops in numerous places in the Northern Territory. During a trip to the Northern Territory in 1974, a prospector visited the Mines Department in Darwin to search through old geological survey reports. In those reports he discovered a reference to outcrops of gemstones in the Wave Hill area of Northern Territory, Australia, some 800 kilometres south south west of Darwin.
Armed with this knowledge he embarked on numerous field trips to the area and in the early 1980's he discovered what is now believed to be the largest deposit of facet grade prehnite in the world. During a field trip to the area, one of the team members, upon cracking open a chunk of basalt exposed a nodule of deep yellow prehnite and exclaimed, "Oh, it looks just like bottled sunlight."
From this statement the name, SunJade® was born to describe this beautiful gem variety of prehnite. After serious experimentation with cutting techniques we have discovered the correct orientation and dimensions to gain the best visual effect from this rare and unique gemstone creating "moonstone" and "cat's-eye" effects. SunJade® can also be faceted and we are only just beginning to see the exceptional potential of this wonderful stone. As we design new and exciting jewellery, SunJade® will gain acclaim on the international market as well as in Australia.
Bibliography and Reference on Prehnite
1. Paul R. Shaffer, Rocks, Gems and Minerals . Martin's Press
2. Swanlake Gems Holdings, Prehnite . www.prehnite.com.au
3. Mindat.org, Classification of Prehnite . www.mindat.org
4. Gerald Pauley, SunJade ® Australian Prehnite . www.openallday.au.com