Diamond Glossary A–Z: Terms Every Seller Should Know

Every word a US diamond buyer might use during a quote, defined in plain English and short enough to read on a phone. Cross-linked to the guides where each concept actually matters.

4 Cs: The four properties that determine a diamond's value on the resale market: cut, color, clarity, and carat weight. Professional buyers grade every diamond on these four scales before pricing it, whether or not a grading report is provided.

A

AGSL: The American Gem Society Laboratories — a US grading lab known for its proprietary 0-to-10 cut grading scale, often considered the most rigorous in the industry. AGSL reports are treated as comparable to GIA by most US buyers.
Appraisal: A written valuation of a diamond — usually for insurance purposes — stating a replacement value at retail. An appraisal is not what a buyer will pay; resale offers typically come in at 25–50% of an insurance appraisal.

C

Carat: The unit of weight used for diamonds — one carat equals 0.2 grams or 200 milligrams. Carat is a measure of weight, not size. Pricing rises sharply at round numbers (1.00, 1.50, 2.00 ct), so a 0.99 ct stone is meaningfully cheaper than a 1.00 ct stone of equal quality.
Certification: A written report from an independent gemological lab grading a diamond against the 4 Cs and other characteristics. On the resale market, GIA reports are the gold standard; AGSL is comparable. EGL and IGI reports exist but are typically re-graded by professional buyers.
Clarity: The grade describing the size, number, and visibility of internal inclusions and surface blemishes in a diamond. GIA's scale runs from Flawless (FL) down to Included (I3). VS1 and VS2 are the practical sweet spot — eye-clean to the buyer, priced well below higher grades.
Color: The grade describing how little yellow or brown tint a diamond shows. The GIA color scale runs from D (completely colorless) to Z (light yellow). On resale, stones in the D–F range fetch a premium; G–J are the practical sweet spot for value.
Cut: The grade describing how well a diamond was shaped and polished — how its proportions, symmetry, and finish return light to the eye. Cut is the most influential of the 4 Cs on visual beauty and one of the biggest drivers of resale value.

E

EGL: The European Gemological Laboratory — a grading lab whose reports are generally considered more lenient than GIA. Professional buyers typically discount EGL grades by one to two steps in color and clarity when calculating a resale offer.

F

Fluorescence: A diamond's tendency to glow blue (or, rarely, another color) under ultraviolet light. Strong fluorescence can reduce a diamond's resale value by 5–15%, especially in colorless (D–F) stones, because some buyers perceive it as a haziness in daylight.

G

GIA: The Gemological Institute of America — the nonprofit research and grading lab whose certificates are the most trusted in the US diamond market. A GIA-graded diamond typically resells for 15–25% more than a comparable uncertified stone.

I

Inclusion: A small internal characteristic — a crystal, feather, cloud, or pinpoint — formed inside a diamond during its growth. Inclusions are how clarity is graded; their size, number, location, and visibility determine where a stone lands on the FL-to-I3 scale.

L

Lab-grown diamond: A diamond physically and chemically identical to a mined diamond but produced in a laboratory using either CVD or HPHT processes. Lab-grown diamonds hold only 10–30% of retail on resale, and that figure continues to fall as production scales.

N

Natural diamond: A diamond formed underground over hundreds of millions of years and mined from the earth. Natural diamonds retain 20–60% of retail value on resale, depending on quality, certification, and brand — significantly more than lab-grown equivalents.

R

Rapaport Price List: A weekly wholesale price benchmark, published by Martin Rapaport, used by diamond dealers worldwide to price stones by shape, weight, color, and clarity. The 'Rap List' sets the wholesale anchor; resale offers are typically expressed as a discount to Rap.
Retail markup: The difference between the wholesale cost of a diamond and the price it sells for in a retail jewelry store. Typical retail markups on diamond jewelry run 100–200%, which is the structural reason resale offers feel low next to original purchase prices.

W

Wholesale price: The price diamond dealers pay each other in the trade — the level at which most resale offers are anchored. Wholesale prices are roughly 30–50% below retail and are tracked through the Rapaport Price List and dealer networks.