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

SKU	GIA Certificate	Carat	GIA Color	GIA Clarity	IGI Certificate	IGI Color	IGI Clarity
2868683	1182317325	0.59	F	I1	3806123461	G	I1
171286	6137937459	1.01	E	SI2	32556249	E	SI2
2524001	7196669648	1	F	SI1	4012131281	F	SI1
209024	5151343118	0.51	D	VS2	m2b31613	D	VS2
145081	2155343246	0.5	E	VS2	f5f87247	E	VS2
2653722	2185231526	0.91	J	VS2	229612184	I	VS2
LD08072817	2185094269	1.94	G	SI2	33280637	I	I1
LD04267055	1152764287	0.59	I	SI1	31993537	G	SI1
LD06548219	1218188877	2.12	I	VS1	s2g89370	H	VS1
LD07995973	5246398539	0.7	H	VS2	31535135	I	VS2
LD04768935	2165485249	0.91	G	SI1	32828352	H	SI2
LD03290846	5151344223	0.53	G	VVS1	m2b37167	H	VVS2
LD08312124	5181078626	0.55	D	SI2	3105265u	F	SI2
LD01850208	2125277385	1.09	F	VS1	m2a82612	F	VS1
LD08150070	6187126800	1	H	SI1	30544666	I	SI1
LD03932535	2151589122	0.38	E	SI2	31217299	F	SI2
LD07646935	5171925166	0.7	G	SI1	30857903	G	SI1
LD07506544	6173816486	0.5	E	VS2	0021142u	F	VS2
LD02431862	2141379411	0.7	E	VVS1	m2b61955	E	VVS2
LD08088215	5126031455	1	E	VS1	m1g27886	D	VS1
LD07536587	1219977788	1.2	E	VVS1	f4b48904	D	IF
LD03334118	5151343407	0.53	H	VVS1	m2b37196	G	IF
LD07553995	1162495357	1	H	SI2	30231524	I	SI2
LD06863914	2175518046	1	E	SI2	33130607	E	SI1
LD08141567	2181128355	0.99	G	VVS1	32540941	G	VS1
LD07274298	2175503514	1.09	I	VS2	3022505b	I	VS2
LD07186726	1176676302	0.9	J	VS1	f6b32887	I	VS1
LD06549221	2175424396	1.14	I	SI1	5813715	I	SI1
LD07536593	1217977810	1.16	D	VVS1	s1g30180	D	IF

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.

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 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.

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 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

Color	Clarity	Carat	IGI Price	IGI N	GIA Price	GIA N	%
D	IF	[0.5,0.6)	3280	9	2935	504	1.12
D	SI2	[0.5,0.6)	1110	27	1300	211	0.85
D	SI2	[0.6,0.7)	1470	7	1660	41	0.89
D	SI2	[0.7,0.8)	2010	5	2260	141	0.89
D	SI2	[1,1.1)	4765	8	5750	149	0.83
D	SI2	[1.5,1.6)	7780	5	10385	26	0.75
E	SI2	[0.3,0.4)	530	5	610	281	0.87
E	SI2	[0.4,0.5)	740	11	810	132	0.91
E	SI2	[0.5,0.6)	1080	31	1230	184	0.88
E	SI2	[0.6,0.7)	1415	10	1510	54	0.94
E	SI2	[0.7,0.8)	1770	23	2160	168	0.82
E	SI2	[0.8,0.9)	2290	7	2770	30	0.83
E	SI2	[1,1.1)	4630	17	5495	186	0.84
E	SI2	[1.5,1.6)	8370	6	10475	42	0.80
F	SI2	[0.5,0.6)	1045	30	1230	163	0.85
F	SI2	[0.6,0.7)	1330	16	1470	53	0.90
F	SI2	[0.7,0.8)	1780	25	2080	179	0.86
F	SI2	[1,1.1)	4015	44	5300	240	0.76
F	SI2	[1.1,1.2)	4415	8	6185	32	0.71
F	SI2	[1.2,1.3)	4770	5	6920	54	0.69
F	SI2	[1.5,1.6)	7260	8	9990	47	0.73
G	SI1	[0.7,0.8)	2155	8	2490	229	0.87
G	SI2	[0.5,0.6)	1060	35	1180	154	0.90
G	SI2	[0.6,0.7)	1320	15	1490	56	0.89
G	SI2	[0.7,0.8)	1760	20	1965	182	0.90
G	SI2	[1,1.1)	3955	52	4950	328	0.80
G	SI2	[1.5,1.6)	7515	6	9210	73	0.82
G	SI2	[2,2.1)	11550	9	15980	70	0.72
G	VVS1	[1,1.1)	7630	7	8435	114	0.90
G	VVS2	[1,1.1)	7230	7	7820	173	0.92
H	IF	[0.7,0.8)	2845	6	2950	190	0.96
H	SI1	[0.5,0.6)	1330	5	1450	130	0.92
H	SI1	[0.7,0.8)	2320	8	2370	158	0.98
H	SI1	[1,1.1)	4640	8	5570	160	0.83
H	SI2	[0.4,0.5)	730	8	760	48	0.96
H	SI2	[0.5,0.6)	980	30	1200	83	0.82
H	SI2	[0.6,0.7)	1325	22	1440	49	0.92
H	SI2	[0.7,0.8)	1690	17	1910	98	0.88
H	SI2	[1,1.1)	3830	35	4715	304	0.81
H	SI2	[1.2,1.3)	4880	7	5960	70	0.82
H	SI2	[1.5,1.6)	6990	11	8710	93	0.80
H	SI2	[1.7,1.8)	8440	7	10690	42	0.79
H	SI2	[2,2.1)	11460	10	14820	84	0.77
H	VS1	[0.5,0.6)	1690	7	1700	211	0.99
H	VS1	[1,1.1)	5760	17	6720	144	0.86
H	VS1	[1.5,1.6)	10450	7	12130	71	0.86
H	VS1	[2,2.1)	18440	7	22200	45	0.83
H	VS2	[0.5,0.6)	1660	13	1600	205	1.04
H	VS2	[1,1.1)	5430	13	6300	145	0.86
H	VS2	[1.5,1.6)	10410	7	11580	92	0.90
H	VVS1	[0.5,0.6)	1580	6	1720	254	0.92
H	VVS1	[0.7,0.8)	2700	8	2870	155	0.94
H	VVS1	[1,1.1)	6140	13	7280	103	0.84
H	VVS1	[2,2.1)	20710	5	26000	11	0.80
H	VVS2	[0.5,0.6)	1570	12	1675	258	0.94
H	VVS2	[1,1.1)	5970	12	6780	109	0.88
H	VVS2	[1.5,1.6)	10680	7	12550	37	0.85
I	IF	[0.5,0.6)	1540	5	1550	189	0.99
I	IF	[0.7,0.8)	2730	8	2720	133	1.00
I	IF	[1,1.1)	5280	6	6160	31	0.86
I	SI1	[0.3,0.4)	670	6	600	105	1.12
I	SI1	[1,1.1)	4270	7	5055	188	0.84
I	SI2	[0.3,0.4)	620	7	550	53	1.13
I	SI2	[0.4,0.5)	680	10	730	34	0.93
I	SI2	[0.5,0.6)	960	53	1080	74	0.89
I	SI2	[0.7,0.8)	1610	5	1760	79	0.91
I	SI2	[1,1.1)	3630	32	4345	200	0.84
I	SI2	[1.1,1.2)	3690	5	4850	48	0.76
I	SI2	[1.2,1.3)	4190	6	5470	72	0.77
I	SI2	[1.5,1.6)	6380	9	8020	74	0.80
I	SI2	[1.7,1.8)	7640	5	9770	37	0.78
I	SI2	[2,2.1)	11100	11	13610	106	0.82
I	VS1	[0.3,0.4)	590	5	630	138	0.94
I	VS1	[0.7,0.8)	2240	5	2420	86	0.93
I	VS1	[1,1.1)	5140	17	5750	89	0.89
I	VS2	[0.7,0.8)	2160	5	2320	110	0.93
I	VS2	[1,1.1)	5140	6	5650	102	0.91
I	VVS1	[0.6,0.7)	1650	8	1790	57	0.92
I	VVS1	[0.7,0.8)	2340	10	2500	122	0.94
I	VVS1	[1,1.1)	5145	20	6020	105	0.85
I	VVS1	[1.3,1.4)	7630	6	8260	13	0.92
I	VVS2	[0.7,0.8)	2220	11	2430	118	0.91
I	VVS2	[1,1.1)	5355	6	5950	71	0.90
I	VVS2	[1.5,1.6)	9450	5	10290	26	0.92
J	IF	[0.5,0.6)	1215	8	1330	153	0.91
J	SI1	[1,1.1)	3795	6	4385	84	0.87
J	SI2	[0.4,0.5)	625	6	700	29	0.89
J	SI2	[0.5,0.6)	930	13	1040	68	0.89
J	SI2	[0.7,0.8)	1430	6	1610	37	0.89
J	SI2	[1,1.1)	3330	23	3990	179	0.83
J	SI2	[1.1,1.2)	3390	11	4400	29	0.77
J	SI2	[1.2,1.3)	4080	6	4990	57	0.82
J	SI2	[2,2.1)	9180	6	11400	37	0.81
J	VS1	[1,1.1)	4270	8	4845	62	0.88
J	VS2	[1,1.1)	4200	11	4765	84	0.88
J	VVS1	[1,1.1)	4740	11	5300	51	0.89
J	VVS2	[1,1.1)	4205	6	4975	54	0.85
K	SI1	[0.5,0.6)	960	9	960	70	1.00
K	SI1	[1,1.1)	3475	8	3610	58	0.96
K	SI2	[0.5,0.6)	820	6	920	58	0.89
K	SI2	[1,1.1)	2800	10	3340	48	0.84
K	VS1	[0.3,0.4)	430	5	520	78	0.83
K	VS1	[0.5,0.6)	975	10	1040	57	0.94
K	VS1	[1,1.1)	3505	6	3920	12	0.89
K	VS2	[0.5,0.6)	1020	7	980	72	1.04
K	VS2	[1,1.1)	3495	6	3710	31	0.94
K	VVS1	[1.5,1.6)	6840	5	7930	11	0.86
K	VVS2	[0.5,0.6)	980	8	1030	37	0.95

Figure 3. IGI vs. GIA Pricing Comparison

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.