Smooth surfaced, non-faceted cuts of gemstones, in any shape.

Cabochons Index

A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U-V  W  X-Y-Z

Malachite - Mohs 3.5-4

A dark green to light green stone that normally has wonderful banding patterns. The color comes from copper and you can often find malachite intergrown with various copper ores. A lot of the malachite I have purchased has come from Africa.

Malachite was popular for jewelry with the Egyptians, Greek, and Romans and remains popular today. It is a soft stone though and does require some TLC!

Marra Mamba

The term Marra Mamba (often mispelled as mara mamba) is now pretty much reserved for the "non-golden" tiger iron from the marra mamba iron formation in Australia. It was mined from isolated pockets that have been mined out, although new deposits could be found. It is richly colored - browns, reds, blues, greens, mustards, silvers - and highly chatoyant.

Also see Tiger Eye.

Mohs Hardness Scale

Developed by Friedrich Mohs, a number is used to indicate the relative hardness or softness of a mineral. It works on the principal that a hard mineral can scratch a softer mineral, but a soft mineral can not scratch a harder mineral. The relative scale accounts for differences in hardness simply by testing which mineral scratches another mineral. A soft mineral like fluorite can be scratched by all of the minerals above it on the scale. The higher up on the scale, the harder and more durable the gemstone is. The lower down on the scale, the softer the stone and the more TLC it requires.

1. Talc
2. Gypsum
3. Calcite
4. Fluorite
5. Apatite
6. Orthoclase
7. Quartz
8. Topaz
9. Corundum (ruby and sapphire)
10. Diamond

Moonstone - Mohs 6.0

Moonstone is a feldspar. It is translucent to opaque and as you move it, a wonderful play of white to bluish-white color glides over the surface. Many sources describe the color as being opalescent. It is normally found in white to cream to brown tones.

(If you like technical terms, moonstone is a potassium feldspar of the orthoclase or adularia species and the color play is called adularescence.)

For pictures & more information, visit the International Colored Gemstone Assoc.

Moss Agate

Agate having random pattern of inclusions, often creating the appearance of seaweed or moss. Moss agate comes in many colors, but is often green.

Opal - Mohs 5.5-6.5

Opal can be
       precious - it has plays of color (the opal that Australia is famous for)
       or 
       common - it does not have plays of color (fire opal, peruvian opal, etc)

Opal is a hardened silica gel with a high water content. It is made up of tiny, transparent, hard silica spheres cemented together by a silica gel. It is the arrangement of the spheres that determines if opal is precious or common. The two forms of opal can be found intermixed.

As water moves - runs down through the earth or bubbles up from hot springs - it can pick up silica and carry it into cracks, crevices, and cavities in rocks, including those caused by decomposing fossils. Over time, the water evaporates, silica spheres settle out, the remaining silica gels, and opal is created. Opal can be found in thin veins, sheets, nodules, or just "sparks" in a host rock. Opal is often found in areas with volcanic rocks and areas with hot springs.


Precious Opal is opal that displays "plays of color". In precious opal, the silica spheres occur in organized pockets, where the spheres are approximately the same size and have a very regular arrangment. The pockets diffract light and create colors - which color and how intense depends on the size and arrangement of the spheres. Diffraction changes with the viewing angle, so colors change and sparks flash out as the stone is moved. "Larger" spheres (a millionth of an inch) produce reds, which is considered a rarer color. Medium spheres produce blues and greens, which are more common colors. The smallest spheres create purples, which is considered a rarer color. All of the color plays are a result of optical diffraction.

In addition to the color plays, there is a background color. This is the color of the silica solution that is cementing all of the spheres together. It seems to be defined both in terms of color and also in terms of light transmission - transparency, translucency, or opaqueness. While pure silica is transparent and colorless (crystal opals), fine impurities (iron, manganese, carbon, etc) in the silica solution give it colors from light colors (white opals) to dark colors (black opals) and gas bubbles and/or water content give the silica solution milkiness or cloudiness.

Precious opal is available as solid cabochons of precious opal, or doublets, or triplets. It can be found in combination with potch. And it can be found in combination with other host rocks - normally ironstone - and called either boulder opal or matrix opal.

I buy a lot of opal that is sold as boulder opal. Technically, I suppose some of it should be called boulder matrix opal. From my understanding, boulder opal is the correct name when opal is a sheet across the surface of the host rock, even if the host rock is poking through the surface of the opal in places. Boulder matrix opal or matrix opal is the correct name when the opal is intimately diffused in the cracks, crevices, and pores of the host rock. To me, it is not a really important difference, as I often see both in the same stone. I tend to refer to any of the stones that are a combination of the ironstone host, potch, and precious opal as boulder opal. In addition, there will often be a name in front of that, telling you where in Australia the boulder opal came from, such as Korite Boulder Opal or Yowah Nut Boulder Opal. The price of the boulder opals/matrix opals seem to be dependent on how much precious opal is showing, the intensity of the precious opal colors, and how pleasing the overall patterns are - some are very scenic!


Common Opal (Potch) is opal that does not have plays of color. Not all common opal is "common" though, including Fire Opal and Blue Peruvian Opal. In common opal, the spheres are irregular in size and there is no order to their arrangement or density in the rock. They can also just be missing and the entire rock is composed of the silica gel. Common opal can be transparent, translucent, or opaque. It can be colorless, white, blue, pink, red, orange, yellow, green - just about any color.

Some of the more notable "common" opals -

• Fire Opals - Orange, to orange-red, to yellow. The best are transparent and suitable for faceting. Many are translucent to opaque. Mexico is the most famous place for fire opal, however they are also found elsewhere. They are also referred to as cherry opals.
• Geyserite - siliceous sinter - deposited around hot springs
• Honey Opal - Honey-yellow, translucent opal.
• Hyalite - Colorless with a strong sheen. Also called glass opal and waterstone. Occurs in volcanic environments where silica deposited at high temperatures. It’s structure is more similar to glass than to the spheres of typical opal.
• Peruvian Blue Opal - opaque to translucent blue to blue-green opal from Peru. My favorites are scenic pieces with dendrites or pleasing inclusions of the host rock.
• Peruvian Pink Opal - opaque pink opal from Peru. My favorites include dendrites.

Opal Fluorite

1) Fluorite impregnated and coated with opal.
2) A fine-grained intergrown mixture of (common) opal and fluorite, in which the degree of purple color is probably an indicator of the fluorite content.
3) See Tiffany Stone.

Opalite

Synonym for Opal. Also see Opal, Common. Also see Tiffany Stone.

Opalized

Replaced by or converted into opal.

Opalized Fluorite

1) Fluorite impregnated and coated with opal.
2) A fine-grained intergrown mixture of (common) opal and fluorite, in which the degree of purple color is probably an indicator of the fluorite content.
3) See Tiffany Stone.

Peacock Ore (Bornite)

Bornite, or Peacock Ore, is a copper ore. These chunks are completely natural. The iridescent colors are basically tarnish. The tarnish layer is very thin and light bouncing between the surface of the stone and the layer of tarnish creates the colors, much like the effect that oil creates on water. Bornite tends to have purple and blue colors of tarnish, but it is often intergrown with chalcopyrite, which also tarnishes and adds greens and yellows.

There are a lot of metaphysical properties associated with peacock ore, but an obvious one is that it brings joy and happiness.

I love peacock ore as rough, chunky pendants on big nubby sweaters in the wintertime. The texture of the stone against the texture of the sweater really works. While they look heavy, on a large chain over a bulky sweater you don’t feel them.

DO NOT put peacock ore in an ionic cleaner. The colors will change faster than you can rescue it! I think it is the metals in the stone. You’ll get a very lovely color that will improve with age - but it won’t be purple and blue any more.

Peanut Wood

Peanut Wood is a petrified wood from the Kennedy Ranges of Western Australia. At some point, the wood washed into the ocean and was attacked by shipworms, which are not worms at all but teredo mollusks. The teredos created tunnels in the wood. The wood sunk to the ocean floor and the tunnels filled in with the silica skeletons of tiny marine plankton as they died (radiolarian sediment). This sediment eventually turned into the light tannish areas of the rock. The remaining wood then also petrified, creating the dark chocolate brown areas of the stone. The contrast of the light tans against the dark, almost-black, browns is quite pretty. While it may technically be called worm wood or teredo wood, peanut wood has a specific color, pattern, and origin that sets it apart from other petrified woods.

Pietersite

Pietersite is found in South Africa (Nambia) and China (Henan Province). It is brecciated, which means that somewhere along the line it got broken up, mixed up, and naturally recemented by silica. The result is that the chatoyancy, instead of going in bands, goes in swirls and other patterns.

The colors are browns, blues, reds, yellows, and sometimes greens. The colors are often intermingled in swirled patterns. The Chinese variety has more red and golden/red combinations.

Also see Tiger Eye.

Plume Agate

Agate having fluffy looking inclusions that appear soft and may resemble feathers, plants, or flowers. The inclusions come in a wide array of colors. Plume agate seems to be named based on the geographic location it came from, although I have purchased some that seems to be named based on the color of the plume.

Potch

See Opal, Common

Purple Opal

See Tiffany Stone

Quartz

A large group of minerals composed of silicon dioxide. There are 2 major sub-groups:

Quartz Species - macrocrystalline (large, single crystals) - includes amethyst, aventurine, rock crystal, citrine, hawk’s eye, prasiolite, smoky quartz, rose quartz, and tiger’s eye. Chalcedony Species - cryptocrystalline (finely grained microcrystals) - includes agate, petrified wood, chrysoprase, bloodstone, jasper, carnelian, moss agate, dendritic agate, sagenitec agate, and plume agate. For pictures & more information, visit the International Colored Gemstone Assoc.