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[Source] Black quartz and tourmaline


#1

I am look to facet black quartz. Does any one have any to sell that
is facet quality? Would like to see pictures of the rough stone.
Does any one know how to best facet a piece of black or green
tourmaline. The green tourmaline is green in one direction and darker
green in another. The darkness may be because of a fracture in the
stone. Do I orientate it a certain way? Does it have cleavage planes?
Also How does one polish the stone? With linde a, cerium oxide, or a
fine diamond powder such as 14,000 grit or 50,000 grit? Is it same
for a black tourmaline. Need to make a Renaissance style ring with a
black stone. Any suggestions as to the stone to facet?

Anne


#2

I have faceted a number of tourmalines, so I may be able to help
with the questions.

Does any one know how to best facet a piece of black or green
tourmaline. The green tourmaline is green in one direction and
darker green in another. The darkness may be because of a fracture
in the stone. 

I have never seen a facetable black quartz crystal. (Dark brown
smoky quartz, yes.)

The darkness seen along the length of a crystal (the “c” axis) is
often encountered in tourmaline, and is caused by dichroism. It may
adversely affect color, but may compensated for by correctly
orienting the stone on the dop and by careful selection of the
cutting design. The best solution is not to use a tourmaline with a
closed c axis. Black tourmaline is not affected.

Do I orientate it a certain way? Does it have cleavage planes? 

Orient for best color. Tourmaline has no cleavage but may be fragile
and even self-destruct during cutting.

Also How does one polish the stone? With linde a, cerium oxide, or
a fine diamond powder such as 14,000 grit or 50,000 grit? 

Tourmaline does not polish well with cerium oxide. I Use aluminum
oxide on a suitable lap (tin, Darkside, etc.) Chrome oxide produces
good results, too. 50K diamond may produce a reasonable polish,
using the correct lap. 14K works for cabochons, but is at best a
pre-polish for faceting.

Is it same for a black tourmaline. Need to make a Renaissance
style ring with a black stone. Any suggestions as to the stone to
facet? 

The usual assumption is that a stone to be faceted is transparent.
This is not true of the typical black stone. It is possible to facet
an opaque or somewhat translucent stone, such as a black onyx or
black spinel, or even black tourmaline, if it is free of fractures.
The stone you select may depend on availability of the size you need
for the ring rather than the type of material.

An opaque material allows cutting the crown only, like a rose cut,
so you can use thin rough.

Good luck.
Dick Davies


#3

I can supply facet grade black tourmaline. in paper thin slivers its
blue or green but in any stone bigger then 5 or 6mm it will be pure
black.

Christopher L. Johnston
http://www.chrisjohnstonphoto.com


#4

Black tourmaline is called schorl, it appears black because of a
molecular abundance or saturation of iron. Tourmaline only starts to
exhibit its wonderful colors when the iron content of the
melt/solution has been depleted allowing other elements to substitute
into the lattice structure in place of the preferential iron.

If quartz is subjected to a radiation source, natural or manmade it
will “smoke” the stronger the source and the longer the exposure the
"blacker" it gets. I have seen irradiated quartz that megascopically
is a pure deep black. If you need a stone for a ring, black onyx
might be a off the shelf choice, otherwise tourmaline is a better
bet.

Christopher L. Johnston
http://www.chrisjohnstonphoto.com


#5
The darkness seen along the length of a crystal (the "c" axis) is
often encountered in tourmaline, and is caused by dichroism. 

Just a clarification, with respect, if I may…

The difference in observed color along the C axis of many tourmaline
crystals is caused by a differential in the polarization of light as
it passes along that axis. This MAY result in a darker color (the
most common situation) but can also result in a lighter color or no
difference at all. To be accurate, dichroism does not CAUSE darkness,
but darkness (or a different color) is the RESULT of pleochrosim, or,
since tourmaline grows in the hexagonal system, dichroism.

My guess is that the author of that excellent post was simply a
victim of the electronic messaging world, as the difference in
statements is small, but important.

Happy New Year and back to the grindstone for me…beginning my 40th
year faceting!

Wayne Emery
The Gemcutter… "Bending Light Since 1975"
www.thelittlecameras.com


#6
To be accurate, dichroism does not CAUSE darkness, but darkness (or
a different color) is the RESULT of pleochrosim, or, since
tourmaline grows in the hexagonal system, dichroism. 

Mr. Emery–If color variation or darkness along one axis of a crystal
of tourmaline is a RESULT of its dichroic nature how is it that the
dichroic property is not CAUSALLY related to the directional
variation in darkness or color? I don’t understand this.

Gerald Vaughan


#7
If color variation or darkness along one axis of a crystal of
tourmaline is a RESULT of its dichroic nature how is it that the
dichroic property is not CAUSALLY related to the directional
variation in darkness or color? 

A gem material can be monochroic, dichroic, or trichroic. What this
means is that with dichroic or trichroic gems the light actually
travels through the gem in two or three different paths. The
difference in the colors of a gem as light travels through, ruby has
red and purple, tanzanite has blue, gray, and clear, is the result
of selective absorption depending on the angle the gem material is
viewed from. Pleochroism is a term that describes what we observe as
the result of light traveling the different axis of a gem.
Pleochroism is not causal, selective absorption is. We see the
effect, not the cause.

Wikipedia:

Pleochroism is caused by the double refraction of light by a
mineral. Light of different polarizations is bent different
amounts by the crystal, and therefore follows different paths
through the crystal. The components of a divided light beam
follow different paths within the mineral and travel at different
speeds, and each path will absorb different colors of light. When
the mineral is observed at some angle, light following some
combination of paths and polarizations will be present, each of
which will have had light of different colors absorbed. At
another angle, the light passing through the crystal will be
composed of another combination of light paths and
polarizations, each with their own color. The light passing
through the mineral will therefore have different colors when it
is viewed from different angles, making the stone seem to be of
different colors.

If a gem is in a forest with no one to observe it, is it pleochroic?

Richard Hart G.G.
Denver, Co.