How Moissanite Is Graded: The GRA Grading Process Step by Step

September 21, 2026

When you purchase a GRA-certified moissanite, the certificate represents a formal evaluation conducted by trained gemologists using standardized equipment and protocols. Understanding what that process involves gives you confidence in what you’re buying and helps you compare stones accurately.

What Is the GRA?

The Gemological Research Association (GRA) is a gemological laboratory that specializes in the grading and certification of moissanite and lab-grown gemstones. Unlike GIA, which primarily grades natural diamonds, GRA developed its grading standards specifically for moissanite — accounting for moissanite’s unique optical and physical properties, including its higher refractive index, greater fire, and hexagonal crystal structure. Each GRA certificate carries a unique identification number that can be verified against the laboratory’s database. See our full certification guide for a comparison of GRA, GIA, and IGI.

Step 1: Stone Receipt and Identification

When a stone is submitted to GRA for grading, it is first logged and assigned a unique case number. The stone’s dimensions are measured using precision calipers: diameter (for round stones) or length × width (for fancy shapes), and depth. These measurements establish the stone’s geometric profile and are used in subsequent carat weight calculation. The stone is also visually examined at this stage to confirm it is moissanite (SiC) and not another material, typically using a thermal/electrical conductivity tester calibrated for moissanite’s known conductivity profile.

Step 2: Carat Weight Determination

Carat weight is determined by precision weighing on a calibrated gemological scale accurate to 0.001 carats. For mounted stones that cannot be removed, carat weight may be estimated using the measured dimensions and the specific gravity of silicon carbide (3.21 g/cm³). One carat equals 0.2 grams. Moissanite’s specific gravity of 3.21 — lower than diamond’s 3.51 — means a moissanite stone of the same millimeter dimensions as a diamond will weigh approximately 8.5% less. GRA certificates report the actual measured weight, not an estimated diamond-equivalent weight, though some retailers separately note the “diamond equivalent size” for consumer reference.

Step 3: Color Grading

Moissanite color is graded using a scale adapted from the GIA diamond color scale: DEF — Colorless. No detectable color under standardized viewing conditions. GHI — Near-colorless. Very slight warmth detectable under magnification or controlled lighting. JK — Faint color. Slight warmth visible to trained observers. Color grading is performed by comparing the stone to a set of master comparison stones under controlled lighting (typically daylight-equivalent, 6,500K) against a white background. The stone is viewed table-down through the pavilion to isolate color. Because moissanite’s high dispersion (fire) can interfere with color assessment, GRA graders use standardized suppression conditions. Moissanite Shine sells exclusively DEF (colorless) stones. See our color grading guide.

Step 4: Clarity Grading

Clarity grading assesses internal inclusions and external blemishes under 10× magnification (the standard loupe magnification for gemological grading). The GRA clarity scale mirrors the GIA scale: FL/IF — Flawless / Internally Flawless VVS1/VVS2 — Very Very Slightly Included (inclusions extremely difficult to see under 10×) VS1/VS2 — Very Slightly Included SI1/SI2 — Slightly Included Because moissanite is lab-grown under controlled conditions, the vast majority of commercial moissanite stones grade at VVS1 or better — meaning inclusions are essentially invisible without magnification. Moissanite Shine sells exclusively VVS1 stones. See our VVS1 clarity guide.

Step 5: Cut Grading

Cut grading evaluates three components: proportions (table percentage, depth percentage, crown angle, pavilion angle), symmetry (alignment and uniformity of facets), and polish (surface finish quality). These are assessed using a combination of visual inspection under magnification and angular measurement tools. The cut grade reflects how effectively the stone returns light — both brilliance (white light return) and fire (spectral dispersion). An Excellent cut grade indicates proportions and symmetry optimized for maximum optical performance. Moissanite’s higher refractive index (2.65–2.69 vs. diamond’s 2.42) means moissanite cut standards differ slightly from diamond cut standards — GRA applies moissanite-specific proportion benchmarks rather than adapting diamond cut criteria directly.

Step 6: Certificate Generation and Laser Inscription

After grading is complete, a certificate is generated bearing the unique stone ID, all four grading results (color, clarity, cut, carat), measured dimensions, and the grader’s verification. Many GRA-certified stones also receive a laser inscription on the girdle — the narrow band at the stone’s widest point — with the certificate number. This inscription is visible under 10× magnification and allows permanent traceability between the physical stone and its certificate. To verify your GRA certificate, use the certificate number on the GRA database. See our GRA verification guide.

What to Look for on Your Certificate

When reviewing a GRA certificate: - Confirm the certificate number matches the laser inscription on the stone’s girdle. - Verify color grade DEF (colorless) for highest optical clarity. - Verify clarity VVS1 or FL/IF for eye-clean appearance. - Check cut grade Excellent for maximum light performance. - Note the measured dimensions and compare to the seller’s listed millimeter size. All Moissanite Shine stones ship with GRA certificates meeting these specifications.