Their origin can be traced to the Sumerian period in Mesopotamia. Seals were used in the Ancient Near East from about 3400BC for over three thousand years, with primarily softer gemstones being used. Harder gemstones were later used to aide the craftsman in cutting more precise detail into the stones that would last.
Romans created intaglios with a wider variety of stones from Egypt and the far-east trade routes: garnets, emeralds, carnelian, chalcedony and amethyst, many being translucent. Emery (consisting of corundum, spinel and rutile) from the Greek island of Naxos was used as an abrasive polishing powder for the past 2000 years.
Other caches of intaglio have been discovered, never having been mounted, proof of a large industry of craftsmen producing the carved gems before orders were even placed. It must have been like going to a modern jewelry store where the pieces are already on display for you to make a selection.
On the low end, prices are very doable... Christies London sold a black jasper intaglio of a huntsman leaning on his staff, with a dog, for £1,188 last year. Portraits and mythological tableaux fetch a higher price. A carnelian ring carved with Alexander the Great in the guise of Hercules, his head draped in a lion’s skin, was sold by Bonhams in London for £55,200. And one of Hercules on a carnelian ring was recently estimated to fetch $120,000-$180,000 at Christie’s New York.
Also, be aware that in the world of buying ancient artifacts, there are forgeries--both stones
and rings can be forged. Deal only with a dealer with a long and respected reputation.
For me, I'd be satisfied with that little red boar in the photo at the beginning of this article... after all, one of the best meals I had in Italy was a wild board stew.