CNC Intaglio Carving Process

Intaglio jewelry is a very specific form of artwork consisting in carving a fine stone to represent a shape, a face, a life event or a religious scene. Such form of art dates back from antiquity, the most ancient intaglio ring discovered by archeologists is estimated to be 3000 years old.

While intaglio in jewelry is not fashion nowadays, there are still people fascinated by this form of art and looking for ancient intaglio rings made of carnelian or onyx stone. Some jewelry carvers specialized in reproducing such ancient intaglio gemstone, and the cost of such artwork usually ranges from $300 USD to $800 USD.

Many people interested in this form of art are not wealthy enough to affort such prices, and while asking us for a solution we had the idea to test a CNC carving method on gemstone, that we are going to present in this article.

We will detail the carving method used, the pros and the cons and suggestion to improve it.

 Quick view of intaglio made with the CNC engraving machine on fine stones to be used in jewelry

The engraving tools

To engrave our fine stones, we use a diamond tip driven by a CNC machine link to a computer using a specific software. In our case, we use an AM30 jewelry engraving machine usually used for engraving silver and gold jewelry.

The diamond tip is a standard one as this is the best option to engrave any surface, so we use it to engrave our gemstone.

View of the AM30 CNC Engraving Machine linked to the computer that commands the moves of the diamond tip

As a reminder, diamond’s hardness on Moh’s scale is 10 and the majority of the stones engraved by this method described in this article have a Moh’s scale hardness around 7.

The advantage of the CNC machine is that it is very precise, and we can repeat the number of passes without losing the details of the engraving.

As you will discover, while the diamond tip is the best option to engrave gemstones, one engraving pass is just not enough to get a proper engraving result.

Engraving Tests 

I have tested this engraving method on a small panel of fine stones:

• Black Onyx, hardness 6-7
• Carnelian, hardness 7
• Malachite, hardness 4
• Lapis lazuli, hardness 5-6
• Amethyst, hardness 7

Here you will find the complete list of gemstone with their indicated hardness.

Onyx intaglio

For this test, we made a logo engraving with initials at the center. The engraving is large, made of thin lines and the engraving is repeated 15 times to allow the diamond tip to engrave deep enough so we can feel the engraving with our finger tip.

 View of the first pass of CNC engraving on this onyx stone

For each pass of the diamond tip, few microns of material are scratched from the surface. Once the dust is cleaned, it can be hard to see the engraving from a distance, this is the reason why we repeat the carving program several times.

Here is the final result after 15 passes.

Here, the engraving is deep enough to be clearly visible and you can feel it with your finger

Here is another onyx CNC carving of initials with only 5 passes.

 You can note that less engraving passes affect the visibility of the engraving

Carnelian Intaglio

Carnelian stones were used a lot during antiquity to make intaglio rings or intaglio jewelry for seal purpose. Archeologists have found a lot of ancient intaglio rings set with carved intaglios representing animals, divinities or objects of everyday life dating back from the romans.

As ants were one of the animals represented on roman intaglio rings, we have used this example to make a comparison of a traditional antique carnelian intaglio with this CNC carving method.

Traditional intaglio is made with tools carving the stone tenth of a millimeter deep so it was possible to make a wax seal with such a stone. On the contrary, the engraving we are able to make with this CNC engraving is flatter and is not usable to make a wax seal, still it is visible to the naked eye.

While the CNC engraving on carnelian stones gives a result, we can visualize that this stone is harder than onyx stone and require more engraving passes to give good results.

The ant engraving on this carnelian stone required 25 engraving passes to give the result shown in the picture. 

It is necessary to remark that while this seems a lot, this process is still faster and much cheaper than a traditional intaglio hand carved.

Here is another comparison of a design engraved on carnelian stone by CNC means and traditional carving made by hand. On the left, the design is made with 30 passes while on the right, the deep engraving of the same design is made by hand with specific tools and this intaglio allows the making of wax seals.

Lapis Lazuli Ingalio

Lapis lazuli was a fine stone used a lot during antiquity by Egyptians and archeologists have found a lot of lapis lazuli intaglio jewelry from ancient civilizations.

Being quite soft, Lapis lazuli can be engraved easily with this CNC process as you can see hereafter. It is possible to engrave easily names, initials or drawings with fewer passes.

On the left, Greek initials are engraved in the center of the Greek decoration while on the right, you can see a griffin being engraved with the diamond tip of the CNC engraving machine

Lapis lazuli is easy to engrave, gives a good result quite easily with a few passes of the diamond tip to get the job done. 

Amethyst Intaglio

Amethyst stone is a common fine stone that is also used for intaglio but that is not common as such nowadays. Being a hard stone and translucent, I had the make the test knowing it would be difficult to get a good result.

Once again we used the ant design to be able to compare the engraved amethyst with the carnelian stone engraved previously.

The first engraving test was made with 10 passes of the diamond tip while the second test (other stone identical to the first one) was made with 30 passes.

As you can see, with 30 diamond tip passes, the result is pretty good. The disadvantage of translucent gemstone is that it can be difficult to see the engraved design because of the refraction of the light going through the stone. 

 After 30 passes, you can feel the engraved design with the tip of your finger.

Turquoise Intaglio

Turquoise is a soft stone that is not used for intaglio usually. I wanted to try our CNC engraving process as this is a common gemstone used in jewelry and I thought the result would be pretty good.

 We engraved a Christian cross, with only three passes, the result is pretty good. You can see the stone is broken at the base, this is not an engraving problem as the stone was already damaged before the engraving.

The engraving went wrong as the stone was not properly set on the CNC engraving machine and I was missing more samples (with a different shape) to make more engraving tests.

Turquoise was engraved easily and I think it gives good results, I will have to try another design engraved on a bigger and flatter stone to confirm it, but I am optimistic with CNC turquoise engraving.

Malachite Intaglio

Malachite is another soft gemstone quite common in jewelry, while it is not so common for intaglio ring making.

This time, I engraved with an initial and I was quite happy with the result after only 15 engraving passes.

Conclusion 

Depending on the gemstone hardness, CNC engraving with diamond tip can be easy to difficult. As we have seen through these examples, 30 engraving passes can be required in the case of gemstones with a Moh hardness of 7 (Carnelian, Onyx, Amethyst).

Beyond the gemstone, other parameters have an impact on the ease of engraving:

• Diamond tip wear, as I could see the improvement changing the diamond tip for a new one
• Spring pressure: the pressure on the spring pushing the diamond tip onto the gem to engrave ( I plan to change the spring for a stronger one to test the difference) 

The result of this engraving process is not the same as the classical intaglio engraving usually made in the jewelry industry, as the depth of the engraving is not the same and the finish is not polished.

But this process is much easier and quicker and cheaper and is a good option for someone with a limited budget looking for an intaglio ring he does not plan to use for wax seal making.