I1, abbreviated from Included 1, is a diamond clarity grade assigned by the Gemological Institute of America (GIA) to stones whose inclusions are obvious to the unaided eye under normal face-up viewing conditions. It sits immediately below SI2 and above I2 on the GIA's eleven-point clarity scale, which runs from Flawless (FL) through Included 3 (I3). An I1 diamond typically contains inclusions — crystals, feathers, clouds, or combinations thereof — that are readily visible without magnification, yet do not so severely compromise transparency or structural integrity as to render the stone unsuitable for set jewellery.

Definition and Grading Criteria

The GIA defines the Included grades collectively as stones in which inclusions are obvious under 10× magnification and, crucially, may affect transparency and brilliance. Within the I range, I1 represents the least severe expression of this category. A trained grader examining an I1 diamond face-up under diffuse lighting will identify one or more inclusions without the aid of a loupe. The inclusions are, however, generally confined in number or positioned such that they do not catastrophically scatter light or create significant dark zones across the table facet. Structural integrity, while not guaranteed, is typically adequate for everyday wear in a protective setting such as a bezel or channel mount.

The boundary between SI2 and I1 is one of the most commercially significant thresholds in diamond grading, as it marks the transition from inclusions that are technically eye-clean (or borderline so) to those that are definitively visible. Stones grading near this boundary are sometimes described in the trade as high I1 or low SI2, reflecting the continuous nature of the underlying clarity spectrum despite the discrete grade labels.

Typical Inclusion Types

Inclusions commonly responsible for an I1 grade include:

• Crystals — mineral inclusions, often other diamond crystals, garnets, or dark graphite grains, large or numerous enough to be seen with the naked eye.
• Feathers — fractures or cleavages that, when oriented toward the surface or positioned centrally, catch light and become visible face-up.
• Clouds — diffuse concentrations of minute pinpoints that, when dense, create a milky or hazy appearance reducing transparency.
• Knots — included crystals that reach the surface, sometimes affecting polish and durability.

The position of inclusions matters considerably within the grade. A single dark crystal beneath the table facet will be far more visually disruptive than a cluster of smaller inclusions near the girdle, even if both earn an I1 designation. Buyers and dealers therefore routinely examine the plotting diagram on a grading report alongside the grade itself.

Optical and Structural Considerations

Eye-visible inclusions in I1 stones can reduce the stone's brilliance by interrupting the internal reflection of light. A feather oriented parallel to the table, or a dense cloud occupying the central crown, may create a visible dark or white patch that diminishes the liveliness expected of a well-cut diamond. Not all I1 diamonds are equally affected: a stone with a single small crystal near the girdle may appear nearly as lively as an SI2, while one with a large central feather may look noticeably dull.

Structural durability is a legitimate concern with feather-type inclusions, particularly those that reach or approach the surface. Ultrasonic cleaning, hard blows, or thermal shock can propagate existing fractures. Gemmologists and jewellers generally advise that I1 diamonds containing significant feathers be set in styles that protect the stone from lateral impact, and that ultrasonic cleaning be used with caution or avoided entirely.

Market Position and Commercial Use

I1 diamonds occupy a substantial segment of the commercial jewellery market, particularly in mass-market and mid-range retail. The price differential between SI2 and I1 can be considerable — often 20 to 40 per cent or more for comparable cut, colour, and carat weight — making I1 an accessible entry point for consumers prioritising size or colour over clarity. In melee (small accent stones), the I1 grade is common because individual stones are too small for inclusions to be individually discerned in a finished piece.

Certain setting styles and design contexts also favour I1 material. Heavily faceted halo settings, pavé work, and bezel mounts can obscure or minimise the visual impact of inclusions. Conversely, solitaire settings with open prong work in larger carat weights will expose an I1 stone's inclusions to full scrutiny, and the grade is generally considered less suitable for prominent solitaires above approximately one carat.

Grading Reports

GIA issues full grading reports (the Diamond Grading Report and the Diamond Dossier) for I1 diamonds of qualifying size. The report includes a clarity plot for full reports, identifying the nature, position, and relative size of inclusions. Other major laboratories — including the International Gemological Institute (IGI) and the Antwerp World Diamond Centre (HRD) — use equivalent or closely analogous grading scales, though nomenclature and grade boundaries may differ marginally. When purchasing an I1 diamond, reviewing the clarity plot and, where possible, examining the stone in person or via high-resolution video remains best practice.

Further Reading

• GIA: Diamond Quality Factors — Clarity
• GIA: The 4Cs of Diamond Quality — Clarity