Internal graining describes lines, streaks or planes inside a diamond that arise from irregularities in crystal growth rather than from the inclusion of a foreign mineral. They are a distinct clarity characteristic, plotted with the standard graining symbol on laboratory reports, and they sit alongside the more familiar inclusion types such as crystals, feathers and clouds.

Origin of graining

Graining records local variations in the diamond lattice that occurred while the crystal was growing. Slight changes in growth rate, the trapping of nitrogen or hydrogen in non-uniform concentrations, or transient strain can produce planes along which the refractive index or transparency differs measurably from the surrounding stone. Under crossed polarisers these planes show up as anomalous strain birefringence; under bright field they appear as faint reflective or hazy lines.

Appearance to the gemmologist

Internal graining is generally subtle. It can look like a faint white or reflective line cutting across the stone, a series of parallel streaks that catch light only at certain angles, or a region of mild haze. Strong graining can be enough to lower a clarity grade because it interferes with light transmission, but most graining is mild and has no transparency effect. Observation requires good loupe technique with the stone tilted across multiple positions; many cases are missed under casual inspection because the lines are visible only over a narrow range of angles.

Internal versus surface graining

Internal graining is plotted as an inclusion characteristic; surface graining is a finish issue plotted as a polish characteristic. The two are sometimes coincident, where a growth plane that produces internal graining also resists polishing in the same direction, but the report distinguishes them and the trade should as well.

Grading impact

On a GIA report, the words "internal graining is not shown" appear under the comments section on stones where the graining does not affect the clarity grade and is not plotted on the diagram. Where graining does carry into the grade, the report notes its presence and may include it in the plotting. Internal graining alone is rarely the limiting characteristic in a stone above the SI clarity range, but in flawless and internally flawless candidates it can be the single feature preventing a higher grade.