Comparing Diamond Certification Labs – GIA vs. IGI Grading and Prices

GIA is often held up as the gold standard of diamond grading while other labs are purported to have looser grading standards. However, there is a lack of hard data to support this claim. Prospective head-to-head comparisons are rare due to the cost of sending diamonds to multiple grading labs. When such studies are available they often have small sample sizes (e.g. RapNet study, N=10 diamonds) that limit their ability to draw conclusions about which lab is best.

Rather than conduct an expensive prospective study to compare grading labs, we conducted a retrospective observational study of dual-certified diamonds. We searched the inventories from Blue Nile and James Allen and identified a set of diamonds that were certified by both GIA and IGI (N=29). Our analysis of dual-certified diamonds suggests that IGI’s grading standards are comparable to GIA, corroborating the results of the RapNet study.

We then conducted a comparative analysis of IGI vs. GIA prices listed on James Allen. For two diamonds of the same carat, color, and clarity grade, an IGI certified diamond is on average 12% cheaper than a GIA certified diamond.

Based on the results of the grading and pricing analyses, IGI certified diamonds potentially represent a good value opportunity.

Methods

Dual Certified GIA-IGI Diamonds

We collected GIA certificates for round diamonds listed for sale on Blue Nile (n=143058 diamonds) and James Allen (n=75624 diamonds). We searched the inscription field of each GIA certificate for references to IGI, EGL, or AGSL. For the Blue Nile set, we found 39 dual-certified diamonds (30 IGI, 7 AGSL, 2 EGL USA). For the James Allen set, we found 11 dual-certified diamonds (7 IGI, 1 AGSL, 2 EGL USA). Due to the small number of GIA-AGSL and GIA-EGL USA dual-certified diamonds, we only analyzed the 37 GIA-IGI dual-certified diamonds.

IGI grading characteristics were manually collected from IGI’s website for the GIA-IGI dual-certified stones. No data was available for 8 GIA-IGI diamonds, resulting in a sample size of n=29 diamonds, listed below in Table 1. We estimated summary statistics of whether IGI was stricter (i.e. IGI assigned a lower grade than GIA) or looser (i.e. IGI assigned a higher grade than GIA) than GIA for color and clarity.

Table 1. Dual Certified GIA-IGI Diamonds

SKUGIA CertificateCaratGIA ColorGIA ClarityIGI CertificateIGI ColorIGI Clarity
286868311823173250.59FI13806123461GI1
17128661379374591.01ESI232556249ESI2
252400171966696481FSI14012131281FSI1
20902451513431180.51DVS2m2b31613DVS2
14508121553432460.5EVS2f5f87247EVS2
265372221852315260.91JVS2229612184IVS2
LD0807281721850942691.94GSI233280637II1
LD0426705511527642870.59ISI131993537GSI1
LD0654821912181888772.12IVS1s2g89370HVS1
LD0799597352463985390.7HVS231535135IVS2
LD0476893521654852490.91GSI132828352HSI2
LD0329084651513442230.53GVVS1m2b37167HVVS2
LD0831212451810786260.55DSI23105265uFSI2
LD0185020821252773851.09FVS1m2a82612FVS1
LD0815007061871268001HSI130544666ISI1
LD0393253521515891220.38ESI231217299FSI2
LD0764693551719251660.7GSI130857903GSI1
LD0750654461738164860.5EVS20021142uFVS2
LD0243186221413794110.7EVVS1m2b61955EVVS2
LD0808821551260314551EVS1m1g27886DVS1
LD0753658712199777881.2EVVS1f4b48904DIF
LD0333411851513434070.53HVVS1m2b37196GIF
LD0755399511624953571HSI230231524ISI2
LD0686391421755180461ESI233130607ESI1
LD0814156721811283550.99GVVS132540941GVS1
LD0727429821755035141.09IVS23022505bIVS2
LD0718672611766763020.9JVS1f6b32887IVS1
LD0654922121754243961.14ISI15813715ISI1
LD0753659312179778101.16DVVS1s1g30180DIF

Pricing

James Allen sells GIA-, AGS-, and IGI-graded diamonds. We collected the prices for excellent cut round diamonds and stratified the prices by diamond color, clarity, and size. Size was stratified in 0.1ct increments, e.g. 0.9-0.99, 1-1.09, 1.1-1.19, etc. Strata having fewer than 5 IGI diamonds were discarded due to small IGI sample size. The median prices for IGI and GIA diamonds were calculated for each strata; the median IGI price was divided by the median GIA price to estimate the percent difference. The percent differences were then averaged across all strata to estimate an overall estimate of IGI prices as a percentage of GIA prices.

Results

IGI vs. GIA Color Grading

Summary results of the color grade comparison are shown in Table 2. IGI was stricter than GIA in 12 cases (e.g. IGI graded a diamond F, while GIA graded it E). IGI was looser in 7 cases (e.g. IGI graded a diamond G color, while GIA graded it H color). Both labs assigned the same grade in 10 cases.

Table 2. IGI vs. GIA Color Grade Comparison
Versus GIA N
IGI stricter 12
IGI same 10
IGI looser 7

All pairwise color comparisons are visualized in Figure 1. The line represents cases where IGI and GIA assign the same color grade. Points above the line are cases where IGI was looser. Points below the line are cases where IGI was stricter.

IGI and GIA assigned the same color grade in 12 cases. IGI and GIA were within 1 color grade of each other in 14 cases. IGI and GIA were within 2 color grades of each other in 3 cases.

Figure 1. IGI vs. GIA Color Grade Comparison

IGI vs. GIA Diamond Color Grade Comparison

IGI vs. GIA Clarity Grading

Summary results of the clarity grade comparison are shown in Table 3. IGI was stricter than GIA in 5 cases (e.g. IGI graded a diamond VVS2, while GIA graded it VVS1). IGI was looser in 4 cases (e.g. IGI graded a diamond SI1, while GIA graded it SI2). Both labs assigned the same grade in 20 cases.

Table 3. IGI vs. GIA Clarity Grade Comparison
Versus GIA N
IGI stricter 5
IGI same 20
IGI looser 4

All pairwise clarity comparisons are visualized in Figure 2. The line represents cases where IGI and GIA assign the same clarity grade. Points above the line are cases where IGI was looser. Points below the line are cases where IGI was stricter.

IGI and GIA assigned the same color grade in 20 cases. IGI and GIA were within 1 color grade of each other in all but 1 case.

Figure 2. IGI vs. GIA Clarity Grade Comparison

IGI vs. GIA Diamond Clarity Comparison Graph

IGI vs. GIA Pricing

The results of the pricing comparison are shown in Table 4.  Averaged over all strata, IGI is approximately 12% cheaper than GIA.

Individual price curves stratified by color and clarity for IGI and GIA diamonds < 1.5ct are shown in Figure 3.  IGI is consistently cheaper than GIA, as seen by the blue dots closer to the bottom of the price curves.

James Allen appears to have a smaller selection of IGI diamonds in the D-F color range, except at the SI2 clarity grade.  Their selection expands to include more IGI diamonds for all clarities in the G-J color range, as seen by the increased prevalence of blue dots.

Table 4. IGI vs. GIA Pricing Stratified by Carat, Color, and Clarity

ColorClarityCaratIGI PriceIGI NGIA PriceGIA N%
DIF[0.5,0.6) 3280 9 29355041.12
DSI2[0.5,0.6) 111027 13002110.85
DSI2[0.6,0.7) 1470 7 1660 410.89
DSI2[0.7,0.8) 2010 5 22601410.89
DSI2[1,1.1) 4765 8 57501490.83
DSI2[1.5,1.6) 7780 510385 260.75
ESI2[0.3,0.4) 530 5 6102810.87
ESI2[0.4,0.5) 74011 8101320.91
ESI2[0.5,0.6) 108031 12301840.88
ESI2[0.6,0.7) 141510 1510 540.94
ESI2[0.7,0.8) 177023 21601680.82
ESI2[0.8,0.9) 2290 7 2770 300.83
ESI2[1,1.1) 463017 54951860.84
ESI2[1.5,1.6) 8370 610475 420.80
FSI2[0.5,0.6) 104530 12301630.85
FSI2[0.6,0.7) 133016 1470 530.90
FSI2[0.7,0.8) 178025 20801790.86
FSI2[1,1.1) 401544 53002400.76
FSI2[1.1,1.2) 4415 8 6185 320.71
FSI2[1.2,1.3) 4770 5 6920 540.69
FSI2[1.5,1.6) 7260 8 9990 470.73
GSI1[0.7,0.8) 2155 8 24902290.87
GSI2[0.5,0.6) 106035 11801540.90
GSI2[0.6,0.7) 132015 1490 560.89
GSI2[0.7,0.8) 176020 19651820.90
GSI2[1,1.1) 395552 49503280.80
GSI2[1.5,1.6) 7515 6 9210 730.82
GSI2[2,2.1)11550 915980 700.72
GVVS1[1,1.1) 7630 7 84351140.90
GVVS2[1,1.1) 7230 7 78201730.92
HIF[0.7,0.8) 2845 6 29501900.96
HSI1[0.5,0.6) 1330 5 14501300.92
HSI1[0.7,0.8) 2320 8 23701580.98
HSI1[1,1.1) 4640 8 55701600.83
HSI2[0.4,0.5) 730 8 760 480.96
HSI2[0.5,0.6) 98030 1200 830.82
HSI2[0.6,0.7) 132522 1440 490.92
HSI2[0.7,0.8) 169017 1910 980.88
HSI2[1,1.1) 383035 47153040.81
HSI2[1.2,1.3) 4880 7 5960 700.82
HSI2[1.5,1.6) 699011 8710 930.80
HSI2[1.7,1.8) 8440 710690 420.79
HSI2[2,2.1)114601014820 840.77
HVS1[0.5,0.6) 1690 7 17002110.99
HVS1[1,1.1) 576017 67201440.86
HVS1[1.5,1.6)10450 712130 710.86
HVS1[2,2.1)18440 722200 450.83
HVS2[0.5,0.6) 166013 16002051.04
HVS2[1,1.1) 543013 63001450.86
HVS2[1.5,1.6)10410 711580 920.90
HVVS1[0.5,0.6) 1580 6 17202540.92
HVVS1[0.7,0.8) 2700 8 28701550.94
HVVS1[1,1.1) 614013 72801030.84
HVVS1[2,2.1)20710 526000 110.80
HVVS2[0.5,0.6) 157012 16752580.94
HVVS2[1,1.1) 597012 67801090.88
HVVS2[1.5,1.6)10680 712550 370.85
IIF[0.5,0.6) 1540 5 15501890.99
IIF[0.7,0.8) 2730 8 27201331.00
IIF[1,1.1) 5280 6 6160 310.86
ISI1[0.3,0.4) 670 6 6001051.12
ISI1[1,1.1) 4270 7 50551880.84
ISI2[0.3,0.4) 620 7 550 531.13
ISI2[0.4,0.5) 68010 730 340.93
ISI2[0.5,0.6) 96053 1080 740.89
ISI2[0.7,0.8) 1610 5 1760 790.91
ISI2[1,1.1) 363032 43452000.84
ISI2[1.1,1.2) 3690 5 4850 480.76
ISI2[1.2,1.3) 4190 6 5470 720.77
ISI2[1.5,1.6) 6380 9 8020 740.80
ISI2[1.7,1.8) 7640 5 9770 370.78
ISI2[2,2.1)1110011136101060.82
IVS1[0.3,0.4) 590 5 6301380.94
IVS1[0.7,0.8) 2240 5 2420 860.93
IVS1[1,1.1) 514017 5750 890.89
IVS2[0.7,0.8) 2160 5 23201100.93
IVS2[1,1.1) 5140 6 56501020.91
IVVS1[0.6,0.7) 1650 8 1790 570.92
IVVS1[0.7,0.8) 234010 25001220.94
IVVS1[1,1.1) 514520 60201050.85
IVVS1[1.3,1.4) 7630 6 8260 130.92
IVVS2[0.7,0.8) 222011 24301180.91
IVVS2[1,1.1) 5355 6 5950 710.90
IVVS2[1.5,1.6) 9450 510290 260.92
JIF[0.5,0.6) 1215 8 13301530.91
JSI1[1,1.1) 3795 6 4385 840.87
JSI2[0.4,0.5) 625 6 700 290.89
JSI2[0.5,0.6) 93013 1040 680.89
JSI2[0.7,0.8) 1430 6 1610 370.89
JSI2[1,1.1) 333023 39901790.83
JSI2[1.1,1.2) 339011 4400 290.77
JSI2[1.2,1.3) 4080 6 4990 570.82
JSI2[2,2.1) 9180 611400 370.81
JVS1[1,1.1) 4270 8 4845 620.88
JVS2[1,1.1) 420011 4765 840.88
JVVS1[1,1.1) 474011 5300 510.89
JVVS2[1,1.1) 4205 6 4975 540.85
KSI1[0.5,0.6) 960 9 960 701.00
KSI1[1,1.1) 3475 8 3610 580.96
KSI2[0.5,0.6) 820 6 920 580.89
KSI2[1,1.1) 280010 3340 480.84
KVS1[0.3,0.4) 430 5 520 780.83
KVS1[0.5,0.6) 97510 1040 570.94
KVS1[1,1.1) 3505 6 3920 120.89
KVS2[0.5,0.6) 1020 7 980 721.04
KVS2[1,1.1) 3495 6 3710 310.94
KVVS1[1.5,1.6) 6840 5 7930 110.86
KVVS2[0.5,0.6) 980 8 1030 370.95

Figure 3. IGI vs. GIA Pricing Comparison

IGI vs. GIA Prices for D Color Diamonds

IGI vs. GIA Prices for G Color Diamonds

IGI vs. GIA Prices for J Color Diamonds

IGI vs. GIA Prices for E Color Diamonds

IGI vs. GIA Prices for H Color Diamonds

IGI vs. GIA Prices for F Color Diamonds

IGI vs. GIA Prices for I Color Diamonds

Discussion

IGI and GIA are consistent on clarity. They assigned the same clarity grade in 20/29 cases and were within 1 clarity grade of each other in all but 1 case. IGI was stricter in 5 cases and looser in 4 cases, suggesting there wasn’t a bias favoring one lab over another on clarity.

There was more variation in color grading.  IGI and GIA only agreed on the color grade in 10/29 cases. They were within 1 color grade of each other in all but 3 cases. IGI was stricter in 12 cases and looser in 7 cases, suggesting a slight bias in favor of stricter color grading at IGI. However, we do not have enough data to probe this bias further; it may be an artifact of small sample size or there might be real differences due to other factors such as grading site (e.g. IGI Antwerp vs. IGI Mumbai), changes in grading standards over time (age of IGI certificate), or differences in the grading lamps used by GIA and IGI.

James Allen’s under-representation of higher clarity IGI diamonds in the D-F color range is an interesting result.  Based on a study from the International Diamond Exchange, IGI produces more G and H diamonds than the industry average, but 42.7% of their diamonds are still graded in the D-F range. IGI also tends to issue more certificates for higher clarity grades than the industry average. We would need to see the joint bivariate distribution of IGI color and clarity grades from the IDEX study to determine if James Allen’s inventory distribution is representative of all IGI diamonds.

This study is subject to a number of limitations.  First, it suffers from selection bias.  All of these diamonds were first graded by IGI and then graded by GIA. Dealers and jewelers choose the grading lab that will maximize their revenue, so other diamonds that are sent to GIA first and then graded by IGI may be qualitatively different.  Second, even though this is the largest publicly available set of dual-certified diamonds, it still has a small sample size of N=29.  The small sample size limits the analysis to descriptive statistics. Third, James Allen’s inventory and our set of dual-certified diamonds are separate samples of all IGI diamonds – the grading and pricing results may not be fully generalizable. Finally, there may be other confounding variables that were not considered (e.g. potential changes in grading standards over time).

In conclusion, we conducted a retrospective head-to-head comparison of GIA-IGI dual-certified diamonds. All diamond grades can be independently verified from the diamond certificates. Summarizing the results:

  • GIA and IGI have similar clarity grading standards
  • There is slight bias in favor of IGI for stricter color
  • IGI has an average 12% price discount compared to GIA

Taking the results at face value, IGI potentially represents a good value opportunity. However, it is important to evaluate each diamond individually to determine if it’s a good deal.