Lapidary Terms Explained: Cabochon

This post is the second in a series of explaining terms used in the lapidary world (the first being around the Mohs Hardness Scale). I thought I would go right back to the basics for this one and explain the word Cabochon.

The term 'Cabochon' originates from the Middle French word caboche meaning 'Head'.

A cabochon is a gemstone, generally a semi precious stone, which has been cut, shaped and polished by a Lapidary into a convex or domed stone with a flat back.

Free Form Designer Cabochons; Left to right they are Picture Jasper, Boulder Opal, Chrysoprase and Green Jasper

Cutting a gemstone to cabochon form is usually a way to cut material that is opaque, as opposed to faceting a translucent stone. In general terms, hardness of a given stone is also taken into account with material rating greater than 7 on the Mohs Hardness Scale being reserved for faceting, rather than cabochon cutting.


Free Form Cabochons in Jewellery; Left to right they are Green Serpentine, Kyanite, Boulder Opal,
Agate and Dendritic Chalecedony

It is widely accepted that their are essentially 2 forms of cabochons; Standard cabochons and free form (or 'designer') cabochons.

Standard 30mm x 40mm Oval Cabochons; Left to right they are Petrified Wood, Jasperlite, Banded Chert and Agate

A standard cabochon is a stone which has been cut into a 'standard' shape such as an oval (as pictured above), square or circle whereas a free form or designer cabochon is a stone cut into an unusual shape without the use of a template. Generally designer cabochons have been designed to take full advantage of what the material has to offer visually, or to allow the cutter some artistic licence with the stone at hand. Cutting a stone cabochon generally involves Diamond wheels (both Metal and resin bonded) or Silicon Carbide wheels, although Diamond has become the preferred method. You can view my How To Guide on cutting stone cabochons by clicking here.

Until next time!

Marble Bar Jasper - A beuatiful but hard to come by material

Marble Bar Jasper is a form of very fine grained silica found just outside the small outback town of Marble Bar, the town taking its name from the now famous 'jasper'.

Marble Bar Western Australia - Located south east of Port Hedland approximately 200km from the Great Northern Highway

The area was first colonized when gold was discovered. The 'Bar', as it is locally known was originally mistaken for marble and for the fact that it blocked the flow of the Coongan River.

The 'Bar' - Marble Bar Western Australia - Photo courtesy of 'The Grateful Dad' on Flickr

The jasper, or more accurately the fine grained silica, is a magnificent mix of red, white and blue/black striped rock. The colours in the stone are as a result of inclusions of mineral impurities such as iron ore causing the red (more commonly known as Hematite), pyrites and carbonaceous matter creating the blue/black tones. The white areas of the stone are more 'pure' forms of silica.

As it is now illegal to fossick and remove the material, doing so carrying a $10,000 fine, it is not unheard of to come across 'old stock' every now and again. The rough shown here was found at the recent Gemkhana show in Albury - I couldn't walk past it! In addition to the material shown here, I am also lucky enough to have a small supply of slabs which my Grandfather collected many many years ago.

Marble Bar Jasper Pendant - $40

Although this material is a jasper, forming part of the quartz family, I have always found it to be quite a bit softer than most other Jasper's that I have cut. In saying this, it still finished with a great shine/polish, and the patterns can be spectacular. You can see in the piece shown here how the stone has fractured, shifted and then re healed to form a very interesting piece of rock.

Marble Bar Jasper Pendant - $30

Marble Bar Jasper Pendant - $30

These two pieces also show the colour variations which are evident within the material.

Swiss Army Knife with Marble Bar Jasper Scales

All in all, Marble Bar Jasper is a beautiful and unique piece of natural geology which is admired by many. Unfortunately not all will have the pleasure of cutting this material, but leaving the natural environment of its origins intact is probably more important. I am one of the lucky ones!

Until next time!

Lapidary Terms Explained: Mohs Scale of Mineral Hardness

Since starting this blog in November last year, I have discussed a number of gemstones and minerals from both a chemical/scientific point of view and from a lapidary point of view. Throughout many of these posts, I have referred to the 'Mohs Scale' or 'Mohs Scale of Mineral Hardness' when discussing the hardness of a given gemstone. As such, I thought it prudent to clarify what the 'Mohs Scale' actually is!

Mohs Scale of Hardness: Image
courtesy of www.thegemchronicles.com

The Mohs Scale (also knowns as the Mohs Hardness Scale or Mohs Scale of Hardness) was developed by the German geologist Friedrich Mohs in 1812 and is widely used by lapidaries to describe the hardness of a given mineral or gemstone. Diamond was, at the time, the hardest material known to exist so is at the top of the list, with Talc being at the bottom.

(Check out this article from newscientist.com for an interesting read on the hardest material known to man)

  

Essentially what the Mohs scale does is describe the scratch resistance of a given material compared to other minerals. To simplify this, referring to the above table, gypsum will scratch talc therefor gypsum is harder than talc. Likewise corundum (sapphire, ruby etc) will be scratched by diamond so is therefor softer.

To find the Mohs classification of a particular gemstone or mineral one needs to find the softest material that will be scratched by the given gemstone, or find the hardest material that will be scratch by the given gemstone. The gemstone is then given a relative number.

To give an example of this, steel will (in general terms) scratch Fluorite, but will be scratched by Apatite, giving steel a Mohs rating of 4 - 4.5

The scale does not refer to the relative hardness of a given material, ie. Corundum is twice as hard as Topaz, but diamond is 4 times harder than corundum. The table below (courtesy of Wikipedia) shows this with the additional column titled 'Abosulte Hardness'.

Mohs Hardness Scale with additional column
titled 'Absolute Hardness'

So from now on, when I refer to the 'Mohs Hardness Scale' when discussing gemstones, you will know what I am on about! 

Feel free to also check out my How-To guide on cutting a stone cabochon and my tutorial on creating gemstone scales for Swiss Army Knives

And if you like the content of this blog, feel free to share!

Until next time!