Introduction
A cloud is not a single inclusion. It is a population — hundreds, thousands, sometimes millions of microscopic pinpoints clustered closely enough that the GIA maps them as a single shaded area on the clarity plot. Individually, each pinpoint is too small to see under 10x magnification. Collectively, they can scatter enough light to turn a diamond from transparent to milky.
This collective behaviour is what makes cloud inclusions unusual among clarity characteristics. A crystal, a feather, a needle — these are discrete features that affect the diamond at a specific point (see Clarity Characteristics). A cloud affects the diamond across a volume. When that volume is large enough and the particle density high enough, the result is not a local flaw but a general loss of transparency.
Not every cloud matters. Many diamonds contain small, sparse clouds with no visible impact. The question is always one of degree: how dense, how widespread, and where in the stone.
Key Points
What a cloud inclusion is
Under the GIA grading system, a cloud is a group of pinpoints — microscopic included crystals — too close together to be distinguished individually at 10x magnification (see GIA Clarity Scale). On the clarity plot, it appears as a shaded red area, distinct from the dot symbol used for individual pinpoints (see Plot and Comments). The shading indicates approximate position and extent, but the plot cannot convey density — the single factor that determines whether a cloud is cosmetically significant.
Each particle scatters a fraction of the light passing through it. At low densities, the scattering is negligible. At high densities, it accumulates — light that should return as sharp brilliance instead arrives softened and diffused.
How clouds affect transparency
The severity of a cloud's visual impact depends on three factors:
Density. A sparse cloud — a few hundred pinpoints loosely clustered — may be invisible without magnification. A dense cloud containing millions of sub-microscopic particles can make the diamond look overtly milky. There is no sharp threshold; the transition from transparent to hazy to milky is continuous.
Distribution. A cloud centred beneath the table — the diamond's primary light-entry window — affects the main viewing path and has maximum visual impact. A cloud near the girdle or in a pavilion corner may scatter light in zones that contribute less to face-up appearance.
Extent. A localised cloud occupying 5–10 % of the diamond's volume behaves like a discrete inclusion. A cloud pervading 50 % or more shifts from local inclusion to systemic transparency problem — see Clarity-Transparency Bridge for how this gap between grade and appearance works.
The 'clouds not shown' comment
The most important transparency clue on a GIA report is a sentence in the comments section:
"Clarity grade is based on clouds that are not shown."
This appears when the grade-setting characteristic is a cloud formation too diffuse to represent on the plot. What it tells you: the dominant inclusion is a type that can reduce transparency, and the cloud is too extensive to plot meaningfully. What it does not tell you: whether the diamond actually looks milky, how dense the cloud is, or whether it sits under the table or near the girdle.
Many diamonds carrying this comment are perfectly transparent — the cloud may be sparse enough to affect the grade under magnification without scattering visible light during normal viewing. The comment is a flag, not a verdict. It tells you that the risk exists. Visual verification is the next step — not avoidance.
"Additional clouds are not shown" is a weaker signal. The grade was set by a different characteristic, but unplotted cloudiness exists. The transparency risk is lower, though not absent in SI1 and SI2 stones where even secondary cloudiness can tip the balance. For a full guide to report comments, see Plot and Comments.
SI1–SI2 cloud risks
Cloud inclusions appear at any clarity grade, but they create the most practical problems in the SI1–SI2 range.
At higher grades (VVS–VS), clouds are typically sparse enough to be graded by their visibility under magnification without affecting transparency. The density required to degrade light transmission would push the grade lower.
At SI1 and SI2, the dynamics shift. The grade tolerates larger and more numerous inclusions. A dense cloud can scatter visible light while remaining within grading parameters — the individual particles are small, the relief is low, and no single characteristic is dramatically conspicuous under 10x. The grade does not penalise cumulative scattering; it evaluates characteristics one by one (see Clarity Grading Factors).
The result: an SI1 diamond that looks milky or hazy, with a clarity grade that does not explain the visual shortcoming. This is where the clarity-transparency gap is widest. See Milky Diamonds for the visual characteristics and Hazy Diamonds for the subtler variant.
Evaluating cloud impact: report, video, and price
No single source of information is sufficient. Combine three inputs:
Report comments. If "Clarity grade is based on clouds that are not shown" is present, plan for visual verification. If neither cloud comment appears and the grade-setting characteristic is a plotted crystal or feather, transparency risk from clouds is minimal.
Video under neutral lighting. Static photographs mask transparency problems — studio spotlights compress the brightness range. In video under daylight-equivalent (D65) illumination, watch for contrast between bright and dark facets. A transparent diamond shows sharp transitions; a cloud-affected diamond shows softer, muted transitions with grey rather than black dark zones. Side-by-side comparison with a known transparent stone at the same grade is the most reliable method. See Light Performance Issues.
Price. A 1.00 ct G SI1 round brilliant priced 15–20 % below comparable stones, with no obvious explanation on the report, may be cloud-affected. Experienced dealers either disclose transparency issues or quietly discount. A meaningful discount combined with an unplotted-cloud comment is strong convergence of evidence.
Practical guidance for Czech consumers
- Read the comments first. The comment section is free information. Look for cloud-related notes before anything else.
- Request video under neutral lighting. Czech consumer protection regulations require sellers to substantiate quality claims. Studio-lit photographs are not sufficient for transparency assessment.
- Do not confuse cloud comments with condemnation. "Clouds that are not shown" means the risk exists and needs verification — not that the diamond is necessarily milky.
- Use the 14-day EU withdrawal right. Inspect under daylight and indoor ambient lighting. Cloud-affected transparency may be less obvious under jewellery-store spotlights and more apparent under softer, everyday conditions.
- Weigh the trade-off. A faintly hazy diamond at a meaningful discount can be a rational purchase — the problem is paying transparent-stone prices for a diamond that performs below its grade.
Summary
Cloud inclusions are clusters of microscopic pinpoints whose impact is collective, not individual. At sufficient density and distribution, they scatter enough light to reduce transparency — turning brilliance soft, contrast flat, and the diamond's visual life muted.
The GIA report comment "Clarity grade is based on clouds that are not shown" is the primary warning sign. Not every such diamond is affected visually, but every such diamond should be verified through video under neutral lighting. The SI1–SI2 range carries the highest risk, because cloud density sufficient to reduce transparency can coexist with grades that suggest a clean stone. Report comments, video inspection, and price comparison are the three tools that close the gap between what the report says and what the diamond does.