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- the Rocks and Minerals Weekly Geo-Class :>

This weeks topic - Cabochon stones

And a reminder that all the class logs are available on the net

the URL is: http://www.dragonlogic.com/classroomA.html

Cabochon stones all have 2 properties in common - they are colorful and they are opaque to translucent.

Properties :>

Most of them are hard but this is not always true. The last two weeks we were talking about the different ornamental stones - those used for statuary, carving, inlay and other ornamental and decorative uses. All of these can and are regularly used for cabachons. Some of the more common ornamental and cabochon stones are the agates, rhodacroisite and rhodonite, onyx, jade, and Lapis. Like the ornamental stones almost every sort of colorful stone has been cut as a cabochon at one time or another - so this discussion is going to be far from comprehensive - just some of the better known or more frequently used materials that we haven't already covered.

But first - What is a cabochon? any ideas?

{An} Well, you say, "Cut as a cabochon", so a way of cutting gems?

Right :> and its one of two basic ways the other being faceted so does that give any additional


{A} rounded or oval shaped stones?

Generally yes - although not always :>

Faceting calls for the creation of flat faces on a transparent stone - to reflect and spread the light as it bounces around inside the stone - like a diamond.

{E} like the different cuts such as baguette?

So Cabochon is really everything else :> - but the best known examples are domed ovals or near ovals. Right hun Baguette, Emerald and Brilliant are the 3 best known faceting styles - and we will talk about them in another class :>

{E} :>

Cabachons can be any shape - I've cut squares and circles, and ovals and hearts among others. They generally have a domed or curved top that is polished - but not always the upper surface is always polished tho :> There are 4 major groups stones in cabachons - the Cameos, the standard stones, the eyed stones and the star stones.


Cameos are normally done in onyx, agate, or Mother-of-Pearl - or in the soft stones like Lapis or Alabaster. They are polished figures cut into the stone - often faces. Actually in a true cameo the stone is cut away leaving the figure and then polished

{A} I did not realize Cameos were considered cabachons

Well we often don't think of them as such.

{HW} That's the one with the face carved out?

But they do belong here really - opaque to translucent, unfaceted but polished, and generally of a domed oval shape right?

* A nods

Right son - or more accurately with the carved face left :>

{A} but it does not have to be a face have seen other designs

If the figure is cut into the stone as a hollow its more properly known as an intaglio

{HW} The only one we have ver seen the stone was light brown with a lighter colored face done in two tone jasper or chalcedony :>

Onyx and the jaspers/chalcedonies are the normal material for cameos and intaglios.

Standard Cabachon Stones

These are the stones most frequently cut into cabachons. They are attractive but inexpensive and are fairly durable. most are some variety of Feldspar or the chalcedonies or massive varieties of quartz.

Feldspars. While such materials as Amazonite and Labradorite are frequently cut as cabachons there are a couple of special materials that are only cut as cabachons. The main one of these is orthoclase or albite feldspar that shows a schiller - a reflection from just beneath the surface that seems to make the stone almost glow. When this is found the stone is cut to show off this schiller and is known as Moonstone because its supposed to "glow" like the moon :>

The next couple of materials are copper carbonates, phosphates and silicates - so they have a green to blue color. Obviously things like Azurite and Malachite are often turned into cabachons but as they are soft they are also fragile. There are 3 harder materials that are cut regularly:

  1. Chrysocolla
    (hydrous copper silicate) ranges in hardness from 2.5 to almost 7 depending on the amount of chalcedony mixed with it - the harder varieties are highly prized :>
  2. Variscite
    (hydrated aluminum phosphate(with copper traces) - variscite is softer (H=4-5) and generally green to green-blue.
  3. Turquoise
    Hydrated copper aluminum phosphate - turquoise is harder (H=5-6) and a light to a rich blue in color. Turquoise is normally found only as a massive material but outside of Amelia VA there is a location where crystals have been found. In fact the image on the website is of the largest single cluster of crystals ever found - and the first. Its now a part of the exhibits at the Smithsonian Museum.

any questions so far?

{E} not yet


(iron oxide) comes in a number of forms - when it is unweathered and massive it is often very solid and may actually be silvery. This is know as specular hematite and it or the solid red is varieties can be cut and polished to create very shiny silvery stones that are also fairly dense.

{An} Which oxide?

The formula is Fe2O3 so it contain only the +3 state An

* An nods.

Occasionally other oxides are cut in similar manner but not frequently.


essentially all types of Chalcedony, and the massive quartz varieties are cut as cabochons. these include the onyxs, agates, jaspers, bloodstones and heliotropes, petrified woods and silicious fossils, jaspers, aventurines, and others. the varieties and forms used cover the full range of polishing posibilities from polished geodes and simple cabachons to sculputes made from multiple polished pieces.


Jade (both Nephrite and Jadeite) is tough and fibrous and this creates problems in their cutting and polishing. It often happens that if worked without care single fibers will be pulled from the stone - either along the length of it or the end tips broken off. This creates a distinctive pitted or "orange peel" effect in poorly done stones, and its a royal pain to get rid of once you've done it :> Gotta go back almost to the start sometimes - or simply junk the stone.The best jades have a rich green translucent color but all shades are possible and many stones will have veins of other minerals in them that create pleasing effects.

* E scrolls back to see what was said about white jade.

actually nothing hun :< you want a few comments?

{E} if you would dad, whitish jade with a few brown steaks in it

ok white jade is probably the rarest form - since the color means that there is no iron in it the lavender and pink jades are almost as rare or possibly even rarer as they are colored by lithium (Lavender) and manganese (pink) with no iron present If you look on the website the jade picture includes a lavender vase and the background block includes pink and white jade :>

{E} ohh oh pink jade yup

So the brown streaks are actually other minerals or veins of material in the jade. The best is found in Burma and has an almost sugary texture or look to it.

{E} sugary? to the touch?

More to the eye hun - the grains are almost that size - like looking at a pile of sugar crystals embedded in plastic <G>

* E takes a closer look

{K} how can you mix the two?

You don't but as the rock forms some sections are leached of the iron and others aren't so a large block can have sections of several colors. I used to have a ring I made for myself that was black jade - with a white vein:>

{E} sounds pretty

it was - too bad it was lost in a house fire :<

{A} aaawww :(

{E} :<


Opals form a separate group - not really minerals at all. They are layers of silica "beads" and water - the size of the beads and the disorder the water layers create cause light to reflect and break up inside creating the fire we see.

{HW} is that why your not supposed to let opals get dried out or they get brittle?

That's right son - if they get dried out the fire goes away and they often do get brittle too.

{K} how can you dry out a rock though, isn't the water trapped in the stone?

{HW} we heard you should soak opals and pearls in oil every once in awhile? is that true?

Actually not in oil - but warm water. In opals its like a silica clay almost - with water drops trapped between layers of beads - heat it and the water drops break into single molecules and evaporate. When you cut and polish it your grinding and sanding it and that heats it up - so you use lots of water and lubricant to keep it cool. If you don't it will haze over and frequently the color play forms a very narrow band so if it goes your lost. There are 3 groups of opals:

White opals - color play against a light or white background

Black opals -color against a dark or black background

Fire opals - clear read-orange with or without an internal play of color. the world's best fire opals come from Mexico.

While the best White and Black opals come from NW Australia - Coober Peddy being one well known location. There is a fine location here in the US for Black opal also - when its open - and that is at Virgin Valley Nevada. Other opal Locations include Idaho and Oregon (white), and Hungary

{HW} shoot to bad we didn't know that.

We are close enough to go sometime sons - if they open the valley to collecting some time - its normally closed off these days.

{HW} why is it closed? to many people tramping around?

Something like that sons - or just the liability problems. I believe it was included in some federal park lands a few years ago and that made collecting illegal. Some of the world's best black opal - but its subject to cracking and drying out

{K} drat


agreed :>

{K} is it a rare opal?

Well - opal without a play of colors is fairly common - the color play is rare and so stones with it are known as precious opal.

{A} what gives fire opal the orange color?

The orange of the fire opals comes from traces of hematite and goethite trapped in the stone.

Other Cabachon Materials

Andalusite twins known as Chiastolite are often turned into cabachons showing the light colored crosses inside the darker surroundings.

{K} that must be pretty

it is I'll try to put a picture up on the website.

Staurolite "fairy crosses" are often polished or tumbled (gently) as well and are special cases of cabachons with unusual shapes.

Organic Materials

Pink, red or black. Today its rare to find and a fake coral in white can probably be found in the stores where you are. I know I bought some nice pieces years ago when I was in Jamaica. Its actually made from ground and pressed shell and reef material not coral. but the pieces are fun if soft :>

A metamorphic rock and the hardest and shiniest of the coals. When its solid it can be cut into fine hard black shiny stones known as Jet. Sometimes these were used as buttons before plastics were invented. Shall we see about making some Pieces for Jett?

{K} what do you use them for

Well today just as jewelry

{K} Ooo, make a necklace for Jett ;)

Amber - is fossilized tree sap and the best is a clear honey yellow with fossils embedded in it and preserved there for millions of years. From there it ranges to dark foamy translucent to nearly opaque forms that are often turned into beads.

Any questions on the organics or the standard stones?

{K} now whenever I see tree sap its in small drips, how can big things like bugs get caught

in it, ( yea I watch too much TV and JP but.. ;)

Think of ants and such on a pine tree - most amber is fossilized pine sap :>

{K} only pine?

well conifers of all sorts - like firs and redwoods too - some of the best pieces I've ever seen actually had small frogs and salamanders in them - and cost tens of thousands of $s. So the ideas from Jurassic Park aren't too far out :> Amber is found in several places but the two best are the Baltic coast (Estonia, Latvia, Lithuania etc) and Haiti

{K} oh

{A} ooohhhh neat


{A} and then there is the mythic "Amber Room"

you want to tell us about that hun :>

{A} well

{K} amber room?

* K sits and listens to anna *WG*

Yeah tell us hun :>

{K} she's speechless *giggle*

{WF} A whole ... Room?

{A} what little I know of is, there was once large panels of amber carved out and used to line walls, the "room" disappeared during WWI ( I think ) and is thought to be in Russia some where

{HW} wow

{A} but no one is sure where it ended up

* K bets R and R will find it someday *G*

{K} that must be awesome

{A} from the descriptions it was beautiful

{HW} probably in Switzerland

{A} or stolen and hidden or sold

I think it was part of one of the Russian palaces and was destroyed or lost during the revolution.

{K} aww ;<

{A} lots of things taken in wars have never been found

{K} such a loss

{A} yes

{A} that sounds right DH

{A} Would prolly be worth millions of dollars if ever found

Hundreds of millions more likely :>

{A} quite likely

{K} that's a great incentive to find it <G>

{A} many have tried

{HW} but it would have to be turned over to the rightful owners or heirs well if its from the palaces there aren't any - except the Russian government.

Ok - any other questions or shall we move on?

{K} no questions from me ;)

{K} ( amazing isnt it)

{P} Go on DH :>

{A} :)

Eyed Stones

The first of these we have already talked about - Tigerseye. The other eyed stones all form in the same way - inclusions in the stone that reflect the light strongly creating a band of more intense reflection if cut properly. The trick is to cut the stone to best show this eye. The best know of the eyed stones is:
Catseye - Chrysoberyl.

{WF} Isn't Tiger's Eye a type of obsidian?

Actually its quartz not obsidian - replacing the asbestos Reibekite either completely (yellow tigertseye) or in part (blue tigerseye - also known as Falcon's eye) - so there is a stone for you boys :>

{K} I think so *G*

{HW} sounds like it

If Chrysoberyl is cut so the long dimension of the cabachon is parallel to the inclusions no catseye is formed but the center line does seem to have a brighter shine that is enhanced by increased doming. If the cut is made so the inclusions are at right angles then you get a yellow oval with a vertical bright bar down the short center axis - just like a cat's eye.

* A chuckles atT, "Heh.. and hey there. :)"

{T} ;)

Other minerals will also create eyes in the same way - one of these is quartz and sometimes garnets will too

any questions?

{P} Not a one :>

Star Stones

{P} Ohh :>

Like the eyed stones these are created by inclusions. But in this case instead of the needles (usually if rutile or ilmenite) being aligned in one direction in the stone in star stones these are aligned in several directions not just one and they are usually aligned with the crystal structure of the main mineral.