Intaglio is a technique when you sculpt into the material instead of doing a raised design. The technique is called Reverse Engraving in modern language. It's done with a hand tool rotating engraver (like a Dremel or such with various tiny bits), and there are many examples coming from Russia and China, usually featuring roses and other floral patterns although the technique stem from ancient Greece.
A decade ago I bought a very beautiful silver pendant sporting a dark amber cabochon with a rose engraved in it. (I never liked amber but always been a sucker for roses..)
The rose looked so lovely and lifelike, and despite that I couldn't really afford it I bought the pendant. We were in a place called "The city of Roses" and I always been buying rose jewelry, so I couldn't help myself.
For years and years on end I wondered how this pendant was made and recently I found my beloved necklace again and started using it. I figured I might actually have the skill to make something like it now, and started to experiment with engraving in acrylics with my goldsmiths tools.
Also, now I'm trying to push past the mere rose pattern and take a walk on the wild side. Here be dragons, goldfish and skulls!