How Much Is Onyx Worth?
Onyx is an affordable, hard-wearing stone — and almost all of the jet-black material you see has been dyed. That’s normal; the trick is paying a fair, dyed-agate price for it.
Onyx is a banded variety of chalcedony (microcrystalline quartz). In the trade, “black onyx” usually means agate that’s been dyed solid black — a practice that goes back to Roman times and is completely accepted. Because the raw material is plentiful and the colour is treated, onyx is one of the most affordable gems. Its value lives in size, the quality of the polish, and the workmanship of the cut, not in scarcity.
What drives the price: size, polish and workmanship
An even, deep black with a clean, glassy polish and no surface pits is what you want. Larger calibrated stones, sharp carvings (cameos, intaglios) and well-matched sets cost more because of the labour, not the rock. Banded varieties — black-and-white onyx or reddish sardonyx — can be slightly more if the banding is crisp and well-used in a carving.
Typical fair prices
Dyed black onyx, mid-2020s retail. Onyx is usually sold per piece; per-carat figures help comparison.
| Form / grade | Typical | Fine / large |
|---|---|---|
| Rough / raw | $1–$5/ct | $5–$10/ct |
| Beads / small cabs | $2–$10/ct | $10–$20/ct |
| Polished jewellery grade | $10–$30/ct | $30–$50/ct |
| Fine carving / cameo | $25–$60/ct | $100/ct+ (workmanship) |
Watch-outs
- Paying “rare gem” prices. Black onyx is abundant dyed agate. If a seller frames plain black onyx as scarce or investment-grade, that’s a marketing red flag — the value is in the cut.
- Glass, obsidian or black spinel confusion. Black glass and obsidian are cheaper lookalikes; black spinel and black tourmaline are different (and pricier) stones. Ask for the material if it matters to you.
- “Onyx” marble for décor. The banded “onyx” sold for countertops and lamps is a calcite/aragonite marble — a different material from gem onyx. Don’t cross-shop them.
- Surface-dyed or coated cheap stone. Most black onyx is dyed throughout (fine), but some pieces are only surface-treated and can wear. Buy a solid, well-polished stone.
The International Gem Society explains that most black onyx is dyed chalcedony (a standard, stable treatment) and that value rests on cut and craftsmanship. GIA classes onyx among the chalcedony quartzes.
Buy onyx priced for its cut, not hype
Our sister marketplace lists size, polish and form plainly — so you pay a fair dyed-agate price and judge the workmanship.
Browse onyx at Minerals Kingdom →FAQ
Is dyed black onyx fake?
No. Dyeing agate black to make onyx is an ancient, accepted, stable treatment. It’s real chalcedony — just don’t pay rare-gem prices for it, because the colour is treated and the material is common.
Why is onyx so cheap?
Chalcedony is abundant and the black colour is produced by dyeing, so there’s no scarcity premium. What you pay for is the size of the stone and the quality of the cutting and polish.
What’s the difference between onyx and sardonyx?
Both are banded chalcedony. Onyx has black-and-white bands (or is dyed solid black); sardonyx adds reddish-brown “sard” bands. Sardonyx with crisp, well-used banding in a carving can be worth a little more.
More value guides
Sources
International Gem Society (IGS) — onyx (dyed chalcedony) value & treatment. Gemological Institute of America (GIA) — chalcedony group. Price ranges are Fair Carat’s synthesis of mid-2020s online retail; verify current dealer prices before buying.Independent gem-value research. We don’t sell stones and sellers can’t buy a better verdict.
Informational only — not a formal appraisal. For insurance or resale, get a certified appraisal.