What specific questions should I ask about a diamond's symmetry and polish?

Symmetry and polish are the two finish grades most buyers glance at and never question. That's a mistake. On a GIA report, they sit under "Finish," and for fancy shapes they're the only cut-related grades you'll get at all, which means you should be asking far more specific questions than most buyers ever bother to. So before you hand over money, here are the specific questions to ask about a diamond's symmetry and polish.

Last March a client named Priya walked in with a 1.48 carat oval she'd bought online. The report said Excellent symmetry and polish. She hated it. Under my loupe, the bowtie was a black band across the middle. The symmetry grade never would have caught that, because a bowtie is about facet angles and depth distribution, not whether the facets line up. Ask the right questions instead.

What to ask about symmetry

These are questions for the seller, but they're also questions you should be able to answer yourself if you get the stone under a loupe.

• "Can I see the diamond under 10x magnification, not just a 360 video?" Look for a centered table, a round girdle, and a culet that isn't off to one side. If the seller won't let you inspect it, walk.
• "What exactly prevented this from being Excellent?" If the symmetry grade is Very Good or lower, the seller should be able to tell you the specific deviation. An off-center table? A wavy girdle? Misaligned facet junctions? Vague answers are a no.
• "For this oval, pear, marquise, or cushion, how pronounced is the bowtie?" The bowtie is not always a symmetry issue, but a serious one can drain a stone. Ask for video in natural daylight, not just studio light.
• "Is the outline symmetrical?" For fancy shapes, uneven shoulders or a slightly lopsided outline will show up once the ring is set and you're looking at it every day.

What to ask about polish

Polish grades describe the surface finish. It's a different problem than symmetry, and it has its own set of questions.

• "Are there any polish lines or burn marks?" Polish lines are fine parallel scratches from the wheel. You usually can't see them with the naked eye, but they can catch light at certain angles. Ask to rotate the stone under 10x.
• "Has this diamond been repolished or recut?" An older stone or an old report might not match what you're buying now. Repolishing can soften facet edges, and that affects how crisp the stone looks.
• "Is there surface graining?" That's technically a clarity feature, but it can make the polish look duller than the grade suggests. Check the comments section on the report.
• "Do the facet edges look sharp?" Dull facet edges are the tell of a stone that's been through a second polish after the report was issued. It matters more than most people think.

The question that matters more

Ask about the overall cut grade if the stone is a round brilliant. An Excellent symmetry and polish pair does not mean the proportions are right. A round stone with Excellent finish grades but a Very Good cut grade is still a stone with bad angles. For fancy shapes, there's no overall cut grade, so symmetry and polish carry the whole load. That should make you more cautious, not less.

Also ask whose report you're reading. For a natural diamond, GIA or AGS. For a lab-grown, IGI is the standard. And check the report date. A five-year-old report on a stone that's been kicking around the trade might be optimistic.

The best symmetry and polish grades in the world won't save a stone with bad proportions, and I've set enough of those to know that the only way to be sure is to ask the questions outright and then insist on seeing the answers under a loupe, not in a website photo.