Susan,
Prehnite is most definitely not quartz or even closely related to
quartz. As has been reliably reported here it is a calcium
aluminosilicate.
If you are selling prehnite on your site and representing it as
quartz, you are in violation of the law. Please correct this.
Thank you.
Wayne Emery
The Gemcutter
Prehnite is a quartz. As such, it is prone to fracture.
First, quartz is not unusually fragile (prone to fracture). At least
not more than it’s hardness would suggest. It does not have, for
example, significant cleavage planes. Hardness of quartz is the
definition of mohs 7, not especially hard, and of course an impact
can chip or fracture most stones if hard enough (even diamond). While
quartz isn’t especially durable, it’s also not unusually fragile.
most often, it simply wears down, getting scratched and dinged in
wear, losing facet edges to wear and tear along with minor edge chips
and the like…
Second, Prehnite is NOT quartz (silicon dioxide). A complex
sillicate, it’s a long way from quartz, both chemically and
structurally, though there can be some resemblance visually,
especially with some of the more transluscent agates or other
chalcedonies. Prehnite is noticably more fragile than is quartz.
From Wikipedia, as good a definition as any:
Prehnite is a phyllosilicate of calcium and aluminium with the
formula: Ca2Al(AlSi3O10)(OH)2. Limited Fe3+ substitutes for
aluminium in the structure. Prehnite crystallizes in the
orthorhombic crystal system. It is brittle with an uneven
fracture and a vitreous to pearly lustre. Its hardness is 6-6.5,
its specific gravity is 2.80-2.90 and its color varies from light
green to blue or white. It is translucent.
Though not a zeolite, it is found associated with them and with
datolite, calcite, 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. It was
first discovered in South Africa by Colonel Hendrik Von Prehn,
an early Dutch governor of the Cape of Good Hope colony. Prehnite
was the first mineral to be named after a person.
Peter Rowe
Prehnite is a quartz.
Beg to differ, Quartz is pure Silicon Dioxide (SiO2); Prehnite is a
Calcium, Aluminum, hydrated Silicate (Ca2Al2Si3O10(OH)2). Quartz has
a hardness of 7; Prehnite has a hardness3 of 6 to 6.5; Quartz has a
RI of 1.530-1.540, Prehnite has a RI of 1.611-1.669; and many more
differences
John
I couldn’t let this one pass:
Prehnite is a quartz. As such, it is prone to fracture. While it is not up there with diamonds in price, neither is it a cheap stone. It can be either green or yellow. I have some yellow faceted roundels on my web site if you want to look at a picture. Click on "gemstone beads" then "quartz beads."
Prehnite most emphatically *is not *quartz! It is a mineral in its
own right, considerably softer than quartz, and nowhere near as
common in nature.
Many correct and descriptive answers to the question have already
been posted.
Dick Davies
Susan,
Prehnite is not a quartz! Quartz has an RI of 1.54, with a hardness
of 7 on the Mohs scale. Prehnite has an RI of 1.61 to 1.63 with a
hardness of 6 to 6.5 on the Mohs scale. The chemical formula for
quartz is SiO2, Prehnite is Ca2Al2Si3O10(OH) + Fe. Clearly Prehnite
is not quartz! If you are selling quartz beads as Prehnite, you have
been misinformed and your are misleading your customers.
Linda McMurray G.G., A.J.P. (GIA)
Proper spelling is prehnite itis NOT a quartz —it is amineral in
its own right.It does not crystallize in the hexagonl system as does
quartz—it is orthrhombic itis denser than quartz and it has agood
cleavage which quartz does not have. It is asilcate mineral but so
are tourmalines, garnets,beryls and many more —this does not make
them a form of quartz. Before blatant assertations are made about
where amineral belongs, check a good scientific reference book such
as Dana’s Mineralogy.
Dave Barclay
C&D Gemcraft Geologist/Gemmolgist
Prehnite is one of those mildly attractive minerals that should not find their way into jewelry....unless of course, you'd like to spend time replacing it constantly. It is much too soft for most jewelry wear, and, because it is not durable is technically not a gemstone. it doesn't fit the definition. Not that will stop anyone, but just be aware it is fragile.
An interesting thread… I have a pair of prehnite earrings (not
made by me) which have been favourites of mine for over 10 years…
so there! People are always asking me what they are and never seem to
have heard of prehnite. It definitely has it’s place in jewellery.
Love the transluscent green of them.
Cheers, Renate