Undercutting Gemstone Cabochons – Minerals and Abrasives – Preforming

by georgeingraham on February 8, 2009

For most Lapidaries, cutting gemstone cabochons has most likely resulted in at least a few undercutting experiences.  For the purpose of this blog entry, I wanted to post a bit about undercutting as related in dealing with combined soft and hard mineral compositions . Different from that of flat spots that result from sanding and or polishing across sharply rounded cabochon corners and or edges as an example.

“Undercutting” occurs when softer grains in the rock grind down faster than the harder grains and you end up with a pock-marked effect – some people call it an “orange peel effect. Look at the way the peel on an orange has hollows in it. Picture the hollows as spots where softer grains ground down faster than the surrounding harder grains. Undercutting happens when you have a lapidary material that consists of more than two minerals where there is a significant difference in hardness or the rate that the material will be removed varies significantly because of the differences in hardness within the mineral. It is the structural characteristics that make certain minerals susceptible to “undercutting” and an “orange peel” surface effect.

The effect is more pronounced when using loose grits because the grits can move about, with both individual grains of grit able to move into attitudes where they have “high spots” that grind into the soft grains of the material, and the grains can pile up into little clumps that also sit higher than the surrounding grains – doing their cutting deeper into the soft grains of the stone being worked. Grinding wheels can have similar difficulties because they tend to wear down unevenly – leaving some high spots and/or grains that will undercut the softer materials in a stone. Using sandpaper reduces the effect significantly because all the grains are glued onto the backing in a relatively level fashion – not much chance for grains of grit to end up sticking up higher, or for clumps of them to form and grind deeper into the stone. But as sandpaper wears you can end up with loose grits on it that will cause undercutting, so it is probably best to change the paper at the first signs of trouble – and not use old, worn sheets at all for any material that is prone to undercutting.

The best way to avoid undercutting in materials prone to this is to use sandpaper instead of loose grits or grindstones. And it may still take you a while to come up with the best combination of sanding grits and polish to get the best results: Not all sandpaper is created equal.

Hard felt polishing surfaces are especially recommended for opal, agate and other materials.

Remember too that no degree of polishing will correct a stone which undercut during sanding.

What about possible solutions?

A preform cutting and grinding guide assembly can be used in conjunction with Graves and Diamond Pacific cabochon machines. The Graves grinding or cutting guide assembly can be integral or attached to a cabbing machine.

The reason I mention this particular tool, is because of of “cams”, undercutting is eliminated because the cab is forced to shape to the contour of the cam. The softer mineral compositions in the stone can not undercut the harder compositions, and the harder compositions also have to conform to the shape of the cams. These cams can be home made into any shape to match desired cabochon preforms from any semi-precious mineral that you expect undercutting issues with. Not to mention of course any other semi-precious material as well.

Here is the limitations though.. It is strictly for preforming. It does not solve dealing with surface undercutting.

Preformer Attachment:


Made by the Graves Company. Take the guess work out of shaping and sizing cabochons and faceted stones. Simply assemble the proper cam, turn the handwheel and the preformer will guide the grinding of the stone to the precise size and shape of the cam. Easily adapts to most grinding and faceting machines. For special sizes or shapes, machined metal or nylon cams can be made in your own workshop.

Here is one of a couple of actual semi-cabbing machines that will deal completely with undercutting for the entire cabochon.

A Colbaugh Semi Cabbing unit. It provides me complete semi-cab forming opportunities. It too incorporates the use of cams ( shown below in first pic ) that allow me to overcome potential undercutting of any semi-precious cabochons.

Photobucket Photobucket>

Another similar unit is the Addexton Semi Cabbing unit. Similar operating procedures as the Colbaugh unit.


I brought these two units up only to offer possible solutions to dealing with semi-precious minerals where undercutting is an issue. Not meant to promote by any means.

One of the many minerals I have ran across where undercutting was unavoidable, as an example was this Azurite/Malachite with Quartz. It matters not what ya try to do, the Azurite/Malachite undercuts the Quartz. For myself it has been these Quartz and or Gem Silica mixed minerals that I have had most all my undercutting problems with.

You can see a finished cab here cut from the slab clearly showing the Quartz and or Gem Silica. The semi-cabbing machine deals well with the combination of the softer Azurite/Malachite prone to undercutting the much harder Quartz veins, leaving a nice smooth surface.

azurite_malachite Photobucket

I have searched high and low online and have yet come across a technique that will aid and or overcome undercutting when working similar materials freehand on a cabbing machine.

One mineral I did run into with undercutting suggestions was for the different types of Jade.

The fibrous texture of jade and variation of hardness are the reasons for its reputation as a problem. Apparently some Jade can be sanded dry and works up beautifully. Other material in its final sanding begins to undercut. When this happens, it is suggested to stop immediately and try a new approach. First, a final sanding on a new piece of 600 cloth. Many times this will correct the difficulty and after a few moments the cabochon will be smooth, free of under-cut or peel, and appear almost polished.

Feel free to comment on minerals that have proven difficult for you to work, and undercut on you.

Also…… Please leave tips and tricks comments that you suggest to overcome undercutting while cabbing freehand.

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