A lot of estate jewellery came in with two small sealed bags of rough blue stone and three identification cards from Indra Gemological Lab. Read at thumbnail size, the cards looked like natural sapphire, and our first pass ranked them the most valuable thing in the whole drop.
Enlarged, the largest card says something else. Every line on it matters, so here it is in full.
What the card actually says
- Report: IGL/2021/04/3033, dated 8 April 2021
- Object: Rough
- Identification: Natural Topaz
- Colour: Blue
- Refractive index: 1.61–1.639
- Shape and cut: Uncut, 30.3 × 24.4 × 14.7 mm
- Weight: approximately 70.20 carats
- Comments: Enhanced
Two more cards from the same lab cover stones of roughly 40 and 49 carats. They haven't been read at the same magnification, so everything below is about the largest card.
The one number that rules out sapphire
Refractive index measures how strongly a stone bends light. It is one of the first readings a gemmologist takes, and it separates these two stones without argument.
The Gemological Institute of America gives topaz a refractive index of 1.619 to 1.627, and sapphire 1.762 to 1.770. The card reads 1.61 to 1.639. That sits squarely in topaz territory and nowhere near sapphire. A gap of more than 0.12 is not a borderline reading or a matter of instrument error. It is a different mineral.
So if a card says "blue" and a seller says "sapphire", look for the refractive index line before anything else. If it starts with 1.6, it is not a sapphire.
The word under Comments
"Enhanced" means the colour has been treated. The card does not say how. GIA notes that colourless topaz is plentiful and is often treated to turn it blue, and that irradiation is one way to produce a sky blue.
Treatment matters twice. It lowers what the stone is worth, and it has to be disclosed. The Federal Trade Commission's jewellery guides (16 CFR § 23.24) say it is deceptive not to disclose a treatment that has a significant effect on a stone's value. When the stone is sold online, the disclosure belongs in the product description.
Why these stones are worth tens of dollars, not hundreds
Four things on that card each pull the value down, and they compound:
- Topaz, not sapphire. Blue topaz is widely available. Sapphire is not.
- Enhanced, not natural colour. Treated colour sells for less than untreated colour.
- Rough, not cut. Seventy carats of rough is not seventy carats of finished gem. Cutting loses much of the weight, and the rough has to be good enough to be worth cutting.
- The lab. Buyers give the most weight to reports from the largest laboratories. A card from a smaller lab is useful, and this one is internally consistent, but it does not carry the same weight.
That is why the stones are not listed on their own at a gemstone price. They stay with the lot they arrived in, a mixed estate jewellery group at $145, and the description names them as blue topaz and carries the lab's "Enhanced" comment.
Reading any gem card before you buy
- Identification names the species. "Natural" there describes the mineral, not the colour.
- Refractive index should fit the species it names. Look the range up. It takes a minute.
- Comments is where treatments live. Words to watch for are enhanced, heated, irradiated, filled and diffused.
- Object tells you whether the weight is rough or a cut stone.
- The report number can often be checked with the issuing lab. If it cannot, weigh the card accordingly.
We misread this card once, at the wrong size. The number that corrected us was printed on it the whole time.
Sources: GIA gem encyclopedia entries for topaz and sapphire; FTC Guides for the Jewelry, Precious Metals, and Pewter Industries, 16 CFR § 23.24.