Judging a Stone: What You Can Check and What You Can't

The gem trade runs on the gap between how a stone looks and what it is. A colorless stone can be diamond, moissanite, cubic zirconia or glass, and the difference is invisible to the naked eye. A red one can be ruby, spinel, garnet or glass. Careful looking won’t close that gap, and any guide that suggests otherwise is doing you a disservice.

What you can do is judge the setting and the stone’s condition, ask the right questions and know which claims need a qualified answer. Here’s the split.

What you can check yourself

Setting security. The stone doesn’t move under very light finger pressure, every prong is present and even, and the stone sits level, not tilted.

Surface condition. Under a loupe or phone macro, look where facet edges meet. On a well-kept stone they’re crisp; on one that’s been knocked around they look rounded, dull or abraded. Check the girdle, the thin band at the widest point, for chips; it’s the most exposed spot.

Cleanliness. Dirt under a stone is common and makes it look duller than it is, and a dark patch under the setting is often just grime. But a stone that won’t clean up may be sitting on a rough or porous backing, not dirty at all.

Evenness across a row. In channel or pavé, all stones should sit at the same height. One lower than its neighbors may be loose, and a missing metal bead is a repair waiting to happen.

How it’s put together. Look for gaps between stone and metal, glue at the edge of the setting, or a seam around the middle of a stone, which suggests it’s assembled from two parts instead of cut from one.

What you can’t check yourself

• Identity. Diamond, moissanite, cubic zirconia or glass; sapphire, tanzanite, iolite or glass. That takes a trained gemologist, not a home tester.
• Natural or lab-grown. Lab-grown stones share the physical and optical properties of natural ones. Separating them takes lab equipment. In the US, the FTC Jewelry Guides cover how lab-created and imitation stones must be described, so ask how the seller has labeled it.
• Treatments and fillers. Heating, oiling, fracture filling, dyeing and irradiation are all used, some routinely. Detecting them takes a gemologist’s equipment and experience.
• Origin. A specialist question, and not something you can read from color.

This matters because look-alike stones can need very different care. An oiled or fracture-filled stone is often damaged by ultrasonic cleaning and heat. If you don’t know what you’re holding, use the gentlest method.

Hardness guides care; it doesn’t guarantee durability

The Mohs scale ranks how easily one mineral scratches another. Figures below are from GIA references:

Two warnings. Hardness isn’t toughness: diamond is the hardest mineral yet cleaves if struck at the wrong angle, and emerald is fairly hard but brittle and often treated, a poor pick for a ring worn roughly. And soft stones aren’t just “delicate”: porous turquoise and lapis absorb oils and cosmetics, and opal reacts to rapid temperature change and drying out.

Treatments: what to ask

Treatments aren’t automatically a scandal. Heating is standard for many stones and accepted trade-wide. Others, like fracture filling a heavily cracked stone with glass-like resin, change how the stone must be handled and cleaned.

Ask specifically: natural, lab-grown or simulant? Treated, and how? Is there a report? A written answer is worth far more than a spoken one, and a seller who won’t write anything down is telling you something.

Reports, and what they’re for

For a stone that matters, an independent gemological lab can examine it. A report describes what the stone is and often whether particular treatments were detected. It doesn’t say what it’s worth and isn’t a valuation. It’s specific to the stone tested at that point in time; a report on another stone, or a photocopy, is no evidence about the piece in front of you.

Valuation is a separate job for a qualified appraiser who sees the piece in person. We don’t give valuations or investment opinions, and a site that offers either from photos deserves suspicion.

Cleaning that follows from all this

For a stone you can’t identify confidently: lukewarm water, a drop of mild dish soap, a soft brush and a lint-free cloth. That covers diamond, sapphire, ruby, quartz and most common stones, provided the setting is sound.

Then subtract:

• Ultrasonic cleaners: no for emerald and other oiled or fracture-filled stones, opal, pearls, amber, coral, turquoise, glued pieces and delicate old settings. Ask a jeweler before running anything unfamiliar.
• Steam cleaners: a workshop tool, same exceptions.
• Ammonia dips and jewelry baths: strip plating and attack porous and organic materials.
• Toothpaste, baking soda, abrasive polishes: scratch metal and can wear a stone’s facet edges.

Diamond is the most tolerant common stone, which is why people clean it so casually. That tolerance belongs to the material, not the price tag. A cheap diamond still scratches against another diamond and still falls out of a worn setting, so check prongs regularly and have flattened ones tightened.

Counter checklist

1. Setting first, stone second. A beautiful stone in a failing setting is a future repair bill.
2. Get identity, treatment and report status in writing.
3. Treat claims of rare origin, “investment grade” or future value as unproven until an independent specialist confirms them on the actual piece.
4. Match care to what you know. If nobody can say what it is, clean it gently.
5. Before a big purchase, see a gemologist; before a big valuation, an appraiser. Different jobs.