Introduction
Internal graining is not an inclusion in the conventional sense. It is not a foreign mineral trapped during growth, nor a fracture from mechanical stress. It is the diamond's own crystal lattice, slightly disordered — growth lines recorded in the atomic structure like rings in a tree trunk, except that in a diamond, those lines were never supposed to be visible.
Under 10x magnification, internal graining appears as faint lines, angles, or curves within the stone. In mild cases, the lines are so subtle that a gemologist must rotate the diamond repeatedly under darkfield illumination to confirm their presence. In severe cases, graining creates a visible haziness or waviness that dulls the diamond's transparency — an effect no amount of recutting can correct, because the irregularity exists at the level of the crystal structure itself (see Transparency Problems).
Graining is one of the less intuitive clarity characteristics. It does not appear as a discrete mark on the clarity plot the way a crystal or feather does. It is often described in the comments section rather than plotted. And its visual impact ranges from negligible to significant, depending on type, density, and position within the stone.
Key Points
How internal graining forms
Diamond crystallises deep within the earth's mantle under extreme temperature and pressure. Under stable conditions, carbon atoms arrange themselves into a perfectly regular cubic lattice. But conditions during growth are rarely stable — shifts in temperature, pressure, or chemistry cause atoms to deposit along slightly altered planes. The boundaries between these zones are internal graining.
Three mechanisms produce it:
Irregular growth. The most common cause. Fluctuations in growth rate create zones with slightly different lattice orientations. The boundaries scatter light differently, producing whitish graining lines.
Plastic deformation. After crystallisation, tectonic forces can bend the lattice without fracturing it — carbon atoms shift along glide planes. This produces coloured graining, typically brown, because the distortion alters how the lattice absorbs light. Brown diamonds owe their colour almost entirely to this mechanism.
Twinning. When a crystal grows along two orientations simultaneously, the boundary between growth directions creates sharp, angular graining that may appear alongside twinning wisps (see Clarity Characteristics).
Types of internal graining
The GIA distinguishes graining by its visual appearance under magnification. Each type signals a different formation mechanism and carries a different risk to transparency.
Whitish graining. Faint, colourless lines that appear as ghost-like streaks under darkfield illumination. The most common type. In most cases, whitish graining is minor: visible under 10x but with no impact on face-up appearance. It is frequently the characteristic that separates an IF from a VVS1, or a VVS from a VS — present but inconsequential.
Coloured graining. Lines that carry a tint, most often brown but occasionally greenish, resulting from plastic deformation. When localised, the impact is cosmetic but limited. When it pervades the stone, it shifts the body colour and introduces an uneven, streaky quality — a diamond with extensive brown graining may appear warmer in certain viewing angles than the colour grade suggests.
Reflective graining. Bright, mirror-like lines that flash when the diamond is rotated. The lattice discontinuity is sharp enough to act as an internal mirror. Less common but more visually disruptive — even a few reflective lines beneath the table can flash independently of the diamond's intended light pattern, competing with brilliance and scintillation.
When graining affects transparency
Most internal graining is cosmetically harmless. The question, as with cloud inclusions, is one of degree (see Cloud Inclusions & Transparency).
Graining begins to affect transparency when it meets two conditions: it is dense enough to scatter a meaningful fraction of light passing through the diamond, and it occupies a significant portion of the stone — particularly the zone beneath the table, which is the primary light-entry window.
Dense whitish graining creates an effect similar to looking through slightly textured glass. Light enters and exits, but the image it carries is softened. Contrast between bright and dark facets diminishes. The diamond does not look milky in the way a cloud-affected stone does (see Milky Diamonds); instead, it looks slightly blurred or wavy — as though the internal structure is interfering with the crisp geometry of light return.
Dense coloured graining adds a tonal shift on top of the transparency loss. The diamond may appear alternately clear and brownish as the viewing angle changes, because graining planes catch light unevenly. This inconsistency is often more distracting than a uniform colour shift.
The critical difference between graining and other transparency-reducing characteristics is permanence. A cloud-related haze can sometimes be mitigated if recut geometry avoids the worst-affected zone. Graining cannot be recut away. It is distributed through the crystal structure, not concentrated in discrete pockets. A diamond with pervasive graining will carry that characteristic through any recut.
Graining on GIA reports
Graining occupies an unusual position in GIA reporting. It is recognised as a clarity characteristic, but it is not always plotted on the clarity diagram the way a crystal, feather, or cloud would be.
When graining is the grade-setting feature, the report includes a comment: "Internal graining is not shown." This parallels the "Clarity grade is based on clouds that are not shown" comment — it tells you the dominant characteristic is structural and cannot be meaningfully represented on the plot (see Plot and Comments). As with the cloud comment, this is a flag, not a verdict. Many diamonds carrying this note have graining that is visible only under magnification with no impact on face-up appearance.
When graining is present but not grade-setting, it may appear in the comments as: "Additional graining is not shown." This indicates graining exists but a different characteristic determined the grade. The transparency risk is lower, though in VS2 and SI grades, even secondary graining can contribute to a subtle loss of crispness.
Graining and the clarity grade. The GIA grades graining by its visibility under 10x magnification, using the same criteria applied to other characteristics: size, number, position, nature, and relief (see Clarity Grading Factors). Faint whitish graining visible only with effort under darkfield illumination typically falls within VVS–VS territory. Coloured or reflective graining that is more readily apparent can push the grade into SI range. But as with clouds, the grade reflects visibility under magnification, not transparency impact during normal viewing — the gap described in Clarity-Transparency Bridge applies here as well.
Practical guidance for Czech consumers
- Read the comments section first. If a GIA report carries "Internal graining is not shown," treat it as you would a cloud comment: the risk exists, visual verification is required. Do not dismiss the diamond, but do not buy without seeing it.
- Request video under neutral (D65) lighting. Graining-related haziness is most visible under diffused, daylight-equivalent illumination. Jewellery-store spotlights mask the wavy quality that graining introduces.
- Watch for inconsistent light behaviour. Reflective graining reveals itself when the stone moves — bright lines flash at angles unrelated to the cut geometry, distinct from regular scintillation.
- Consider coloured graining in near-colourless grades. A G or H colour diamond with brown graining may appear warmer than the grade suggests. If colour consistency matters for matching side stones, ask whether graining is noted.
- Use the 14-day EU withdrawal right. Inspect under daylight, indoor ambient, and directed spotlights. Graining-related transparency loss is more visible under soft, even lighting than in retail environments.
- Understand what you cannot change. Unlike polish haze (see Poor Polish Haze), internal graining cannot be resolved by recutting or repolishing. Price should reflect this permanence.
Summary
Internal graining is the diamond's crystal structure made visible — growth irregularities, deformation planes, and twin boundaries recorded in the atomic lattice. It appears under magnification as whitish, coloured, or reflective lines, and in most diamonds it is a minor clarity characteristic with no impact on visual performance.
When graining is extensive, however, it can reduce transparency in ways the clarity grade does not fully capture. Dense whitish graining softens the diamond's light return. Coloured graining introduces tonal inconsistency. Reflective graining creates distracting flashes that compete with the intended brilliance pattern. Unlike cloud-related haziness, graining is structural and permanent — it cannot be polished or recut away.
The GIA report comment "Internal graining is not shown" is the primary indicator. As with cloud comments, it signals the presence of a characteristic that may or may not affect the diamond visually. The only reliable assessment is visual inspection under neutral lighting, with attention to contrast, colour consistency, and the behaviour of light as the stone rotates.