An internal inclusion is a clarity characteristic that lies wholly inside a gemstone and does not reach the polished surface. The category covers crystals, clouds, feathers, needles, growth zones, healing fractures and any other feature that the diamond and coloured-stone grading systems plot inside the outline of the stone rather than on its profile.

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The internal versus external distinction

Diamond grading separates clarity characteristics into two groups: inclusions, which are internal, and blemishes, which are surface features such as scratches, naturals and extra facets. The boundary is sharp. A feather that breaks the surface is plotted as both an inclusion and a blemish, but a feather entirely inside the stone is purely internal. The same logic applies in coloured-stone grading and laboratory clarity descriptions.

Why internality matters

An internal inclusion is locked into the stone and cannot be polished away without recutting and weight loss. It is therefore a permanent feature of the gem. A surface blemish, by contrast, can often be removed by repolishing. The grading distinction reflects this practical difference; a stone that is otherwise flawless but for a small surface scratch can be brought to a higher grade by repolishing, whereas a stone with an internal inclusion of equivalent size cannot.

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Common internal inclusion types

The trade vocabulary for internal inclusions is large and largely standardised. Crystals are mineral inclusions trapped during growth. Clouds are dense aggregations of tiny inclusions that affect transparency. Feathers are internal fractures that may or may not reach the surface. Needles are slender crystal inclusions, often rutile or other oxides. Pinpoints are tiny inclusions too small to identify, plotted as dots. Growth lines, twin planes and graining record structural rather than foreign-material features. Each has its own plot symbol on a laboratory report.

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