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Grading Methods

How GEM examines and grades

Our grading follows internationally recognized terminology. Here is how we describe each category, and how to read the results on your report.

The 4Cs of a diamond

Color

Most diamonds range from colorless to light yellow or brown. We compare each stone with master stones under controlled daylight-balanced lighting and assign a grade from D to Z. Fancy colors such as pink, blue or vivid yellow are described separately.

Color scale

DEF
Colorless
GHIJ
Near colorless
KLM
Faint
NOPQR
Very light
STUVWXYZ
Light

Clarity

Clarity describes internal inclusions and surface blemishes seen at 10× magnification. Their size, number, position and visibility determine the grade.

Clarity scale

FL
Flawless
IF
Internally flawless
VVS1VVS2
Very very slightly included
VS1VS2
Very slightly included
SI1SI2
Slightly included
I1I2I3
Included

Cut

Cut describes how well a diamond's proportions, polish and symmetry return light. For round brilliants we grade from Excellent to Poor; for other shapes we report polish and symmetry.

Cut scale

ExcellentVery GoodGoodFairPoor

Carat weight

Measured on a calibrated electronic balance to the hundredth of a carat. One carat is 0.2 grams. Stones set in jewelry are estimated by formula.

0.25 ct≈4.1 mm
0.50 ct≈5.2 mm
1.00 ct≈6.5 mm
2.00 ct≈8.2 mm
3.00 ct≈9.4 mm

Shapes and cutting styles

The shape describes the outline of the stone; the cutting style describes its facet arrangement.

Round
Oval
Pear
Marquise
Heart
Cushion
Princess
Radiant
Emerald
Asscher

Proportions

Every GEM diamond report includes a proportions diagram: table size, crown and pavilion angles, depth, girdle and culet, measured with a proportion scope. Together they decide how the stone handles light.

Table 57%Girdle Medium FacetedDepth 61.6%Crown angle 34.5°Crown height 15.0%Pavilion angle 40.8°Pavilion depth 43.0%Culet None6.45 - 6.48 x 3.98 mm

Natural or laboratory-grown

Laboratory-grown diamonds have the same chemical, physical and optical properties as natural diamonds. They are produced by CVD (chemical vapor deposition) or HPHT (high pressure, high temperature). Only laboratory instruments can tell them apart, and every GEM report states the origin clearly.

Colored gemstones

For sapphire, ruby, emerald and other gemstones we identify the species and variety, determine whether the stone is natural, synthetic or an imitation, and disclose any treatment we detect, such as heating, oiling, diffusion or filling.

Jewelry

For a finished piece we verify the metal (by XRF where possible), weigh the piece, describe the center and side stones and estimate the total carat weight as the mounting permits.

Metal fineness

The hallmark states the gold or platinum content in parts per thousand.

MetalHallmark%
24K Gold99999.9
22K Gold91691.6
21K Gold87587.5
18K Gold75075.0
14K Gold58558.5
10K Gold41741.7
9K Gold37537.5
Platinum95095.0
Sterling Silver92592.5

Luxury watches

We open and inspect the movement, check the reference and serial against the production period, measure timekeeping on a timegrapher, test water resistance and examine every component for originality before stating a final result.

How to read your GEM report

GEM tri-fold report
  1. 1Report number, barcode and QR code: the link to the original record.
  2. 2Proportions diagram and photograph of the examined item.
  3. 3Grading results and additional characteristics.
  4. 4Grading scales with your item's grade highlighted.
  5. 5Digital fingerprint: changes if any detail of the record is altered.
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