John Rhoads - The Good, the Bad, and the Ugly in Rare Gems
by Nancy Attaway
Award-winning facetor, John Rhoads of D & J Rare Gems, Ltd. in Salida,
Colorado addressed the New Mexico Faceters Guild on cutting rare and difficult
gems. His talk focused on cutting “The Good, the Bad, and the Ugly” in rare
gems. John has been a mineral collector since he was three years old and
was partial to the glassy crystals that he saw. John earned a B.S. in Geology
in 1975 from Kutztown College in Kutztown, Pennsylvania, between Reading
and Allentown. He earned a Master’s in Geology in 1977 from the New Mexico
Institute of Mining and Technology in Socorro. John was involved in uranium
exploration and teaching and lived in several African counties. He earned
a graduate gemological certification from GIA in 1985. His first faceting
machine was a RayTech.
John discussed cutting two types of rare gems, those seen in jewelry and
the more exotic ones normally purchased for specialty collections. He divided
them into three groups, the “Good”, the “Bad”, and the “Ugly”, and he placed
six different gems into each group. Other gems could have been added, but
John chose the six that were the most memorable to him. The “Good” included
those gems that were rare gems but cuttable. The “Bad” represented those
gems that were both rare and a challenge to facet. The “Ugly” marked those
gems as rare and extremely difficult to cut.
In the “Good” category, John placed taaffeite, phenakite, axinite, epidote,
danburite, and euclase. He discussed each one separately and offered comments.
John said that taaffeite is a beryllium magnesium aluminate with a hardness
of 8 to 8.5. John cuts taaffeite on a worn 600-grit lap, pre-polishes it
on a newbond lap, and polishes it on a tin lap with linde A. Taaffeite occurs
in gray, pink, and purple and can also be colorless. Taaffeite has been compared
to the cutting characteristics of spinel.
John said that phenakite is a beryllium silicate with a hardness of 7.5 to
8. Phenakite shows nice terminations and crystal faces. He said phenakite
acts like beryl when faceted, except that phenakite has distinct cleavage
in two directions. John cuts it like he would beryl and polishes it with
either linde A or cerium oxide on a tin lap. Phenakite, which has been confused
with quartz, occurs in straw yellow and pink, and it also can be colorless.
Phenakite is sensitive to ultraviolet light and will fade when exposed to
sunlight over time. Phenakite can be a very brilliant faceted gemstone.
John said that axinite is a calcium aluminum boro-silicate with a hardness
of 6.5 to 7. Axinite is bladed much like the head of an axe and can show
a purplish flash. He remarked that axinite is trichroic and has several color
orientations. It also has a distinct cleavage in one direction. John polishes
axinite on a tin lap with Linde A. He cautioned faceters to watch the lap
speed when polishing axinite and not to go too fast. Axinite can also be
polished with cerium oxide on a lucite lap.
John said that epidote is a calcium iron aluminum silicate with a hardness
of 6 to 7. Epidote is very dark but easy to polish, providing that the facetor
renders a good pre-polish. Epidote has perfect cleavage in two directions
and is also trichroic. John polishes epidote on a tin lap with linde A.
John said that danburite is a calcium borosilicate with a hardness of 7.
Danburite has a high refractive index and usually is colorless. Danburite
can show a slight pink hue and can also be root beer colored or even apricot.
John polishes danburite on a tin lap with linde A. Danburite makes a brilliant
gemstone because of its clarity. It is usually not a problem to facet.
John said that euclase is a beryllium aluminum silicate with a hardness of
7.5. Euclase cleaves perfectly in one direction, and the cleavage may separate
during dopping. Euclase also has twinning planes that will separate from
thermal shock. Euclase occurs in pale blue and pale yellow but can also be
colorless. John polishes euclase on a tin lap with linde A.
In the “Bad” category, John placed apatite, sphene, diopside (including chrome
diopside), fluorite, calcite, and rhodocrosite. He discussed each one separately
and provided comments. John said that he uses a worn 600-grit lap to cut
many of these very brittle gemstones.
John said that apatite is a variable calcium phosphate with a hardness of
5. John remarked that apatite is a tricky gemstone to facet, and that it
can shatter when cut. Apatite is very heat sensitive and can be thermally
shocked. John recommended cold dopping for apatite. He polishes apatite on
a tin lap with a thin slurry of linde A. Apatite is very susceptible to aggregation,
the build up of polishing compound that can cause scratches. It is also susceptible
to flow scratches during the polish stage.
John said that sphene is calcium titano-silicate with a hardness of 5 to
5.5. Sphene is a greenish-brown-yellow color with a high refractive index.
With its high dispersion, sphene is a very eye catching gemstone. John begins
faceting sphene with a 260-grit lap and works through the grits to the polish
stage. John said to orient the table parallel to the twinning plane. He polishes
sphene on a tin lap with a thin slurry of linde A and recommended slow lap
speeds. He said that sphene polishes easy but said to be mindful of aggregation,
the build-up of polishing compound on the lap that can scratch or leave a
John said that diopside is a calcium magnesium silicate with a hardness of
5 to 6. He stated that chrome and chrome diopside were both very brittle
gemstones that can give the facetor problems during polish. If scratching
or chipping problems occur when polishing chrome or chrome diopside, then
the facetor may need to change the direction of the lap. The facetor might
even change the direction at where the gem touches the circle of the polishing
lap. John polishes chrome on a tin lap with linde A. Slow polish speeds are
John remarked that fluorite is a calcium fluoride with a hardness of 4. Fluorite
has perfect cleavage in four directions and is brittle. He said to look for
cleavage planes and grind them down. The cleavages in fluorite can be problematic
during the dopping stage, the cutting stage, and the polish stage. He recommended
orienting the table away from all cleavages. Ten degrees away from the cube
face would work fine. John polishes fluorite on a tin lap with linde A. He
recommended using a hard wax lap to obtain a polish on those facets that
are harder to polish. A wax lap will produce a scratch-free surface but will
tend to round the facet edges. A metal lap will give sharp facet meets but
can cause scratches. Fluorite can also be polished on a chrome oxide Ultralap.
John said to cut fluorite for its lovely color and not so much for brilliance.
John said that calcite is a calcium carbonate with a hardness of 3. Calcite
has perfect cleavage in three directions and is very heat sensitive. He recommended
placing the table at the junction of two cleavage planes. John polishes calcite
on a wax lap with linde A and is very careful with the lap speed and the
pressure. He also recommended keeping the facets small when cutting and polishing
calcite. Calcite can also be polished on a chrome oxide Ultralap. The cutting
and polishing of calcite must be directional with the cleavage.
John stated that rhodochrosite is a manganese carbonate with a hardness of
3.5 to 4.5. Even though rhodochrosite has perfect cleavage in three directions,
it is a bit more robust than calcite. John polishes rhodochrosite on a wax
lap with linde A. He remarked that rhodochrosite was mechanically sensitive
and to be mindful of the pressure and speed when cutting and polishing rhodochrosite.
Rhodochrosite can also be polished on a chrome oxide Ultralap. Inclusions
within the gemstone does not necessarily detract from rhodochrosite’s beautiful
John also mentioned sphalerite, a zinc sulfide with a hardness of 3.5 to
4. Sphalerite has perfect cleavage in six directions and is a very challenging
gemstone to facet. The very high refractive index of sphalerite makes it
a glittering gemstone. John said that the hardest aspect to faceting sphalerite
was orienting the table NOT parallel to a cleavage direction. He uses either
a tin lap or a wax lap with linde A to polish it.
In the “Ugly” category, John placed thaumasite, cuprite, cinnabar, crocoite,
wulfenite, and cerrusite. He discussed each one separately and made observations.
John said that thaumasite was more brittle than most gemstones. He thought
that thaumasite rough resembled small yellow beryl crystals. He remarked
that faceting thaumasite was like faceting a potato chip. John uses a worn
1200-grit lap to cut thaumasite and a wax lap with either diamond or linde
A for polish. He said that thaumasite was very heat sensitive and to go easy
and slow. He has faceted two thaumasites so far.
John said that cuprite is a copper oxide with a hardness of 3.5 to 4, but
that some references have the hardness of cuprite at 2. He said that cuprite
is also photo sensitive and has one direction of cleavage. John polishes
cuprite on a very soft wax lap and goes easy and slow. He remarked that laps
used for cutting and polishing cuprite should only be used exclusively for
cuprite, as cuprite pastes and coats the laps with a glaze. Since cuprite
is very delicate and scratches easily, John gently places a faceted cuprite
in cotton inside a gemstone paper. Simply wiping a faceted cuprite with a
cloth can generate scratches. Cuprite is best cut as a tablet, due to its
very deep, intense red color.
John said that cinnabar is a mercuric sulfide with a hardness of 2 to 2.5.
Cinnabar has one direction of cleavage and is very heat sensitive. Since
cinnabar is also very brittle, John uses very slow lap speeds to cut and
polish the gem. He uses a wax lap to polish cinnabar. He said to clean a
faceted cinnabar gently with alcohol on cotton and not to use a cloth to
John said that crocoite is a lead sulfate with a hardness of 2.5 to 3. Crocoite
has one direction of cleavage, is very heat sensitive, and is extremely brittle.
John polishes crocoite on a soft wax lap with linde A.
John said that wulfenite is a lead molybdate with a hardness of 2.5 to 3.
He said that wulfenite is a very fragile gem that is extremely pressure sensitive.
It also has one direction of cleavage. When working with wulfenite, John
dops the entire piece of rough and backs the entire stone with wax because
it can bend. He polishes wulfenite on a wax lap with linde A.
John said that cerrusite is a lead carbonate with a hardness of 3.5. Cerrusite
has a high dispersion but is extremely heat sensitive. It also has two distinct
directions of cleavage. John cold dops cerrusite rough for faceting. He remarked
that cerrusite has cleavage and that, remarkably enough, it can heal itself.
When John noticed a cleavage plane that had developed on a piece of cerrusite
he was faceting, he stopped and waited until the fracture healed. He then
returned to facet the stone. John said that barite behaves the same way as
cerrusite. John also mentioned the gem, anglesite.
John said that anglesite is a lead sulfate with a hardness of 2.5 to 3. It
has two perfect directions of cleavage. Anglesite is extremely brittle and
very heat sensitive. John said to polish anglesite on a wax lap with linde
A. A chrome oxide Ultralap also works.
John listed several other gemstones that he thought were nightmares to facet.
These nearly-impossible-to-facet gemstones included selenite, lepidolite,
vivianite, halite, realgar, and sulphur. Selenite is gypsum, a calcium sulfate
with a hardness of 2 and three directions of cleavage. Lepidolite is a potassium
lithium aluminum silicate with a hardness of 2.5 to 3 and one perfect cleavage.
Vivianite is an iron phosphate with a hardness of 1.5 to 2 and one direction
of cleavage. Orient vivianite the table parallel to the cleavage plane and
cut and polish with the cleavage, not against it. Halite is sodium chloride
(salt) with a hardness of 2 and two perfect directions of cleavage. Since
halite dissolves in water, polish is rendered with pure grain alcohol. Realgar
is arsenic silicate with a hardness of 1.5 to 2. Realgar is very heat sensitive
and may be the most brittle of all gemstones. Realgar is also light sensitive
and will decompose. Sulphur is pure sulphur with a hardness of 1.5 to 2.5.
Sulphur is extremely brittle and dissolves.
John remarked that Art Grant and Mike Gray of Coast to Coast Rare gems also
have a significant amount of experience faceting rare gemstones, as well
as the gemstones that are very difficult to facet. John mentioned that Mike
Gray was researching the hardness direction of gemstones and wanted to quantify
and measure the actual directional hardness. That study would be interesting
and very challenging to undertake.
John Rhoads presented an excellent discourse on cutting rare and difficult
gemstones. His explanations on how to cut some of the very difficult gemstones
found in nature were insightful. The New Mexico Faceters Guild thanks John
Rhoads for a great talk.