Diamond inclusions

GIA explains that small crystals become trapped in a diamond as it grows, and that pressure and stress during growth can cause inclusions and blemishes. Its graders give each feature a name, with examples such as feather, crystal, cloud and bearded girdle.

The four below are ones buyers meet often. They behave very differently, which is why two stones with the same clarity grade can look and wear differently.

GIA draws the line at the surface. Features inside the diamond, and features that reach the surface from inside, are inclusions. Features that exist only on the outside are blemishes. Scratches and nicks are typical blemishes, and the International Gem Society notes that they can usually be polished away. An inclusion sits inside the stone, so polishing the surface does not remove it.

Clarity grades weigh both kinds. GIA's graders look at the number, size, relief (how strongly a feature stands out against the diamond), nature and position of each feature at 10x magnification. The scale has 11 grades, from Flawless to Included (I3). In the VS grades the inclusions are minor and seen with effort at 10x. In the SI grades they are noticeable at that magnification. In I1 to I3 they are obvious and may affect transparency, brilliance and durability. GIA also says most diamonds in jewellery shops fall between VS and SI, so some inclusions are normal in a diamond bought for a ring.

Sometimes, and position matters. GIA says that in the I1 to I3 range inclusions may affect durability. For feathers, the International Gem Society points to location: tiny feathers along the girdle can sit in a stone with a high clarity grade, while a large feather may cause the stone to crack and pulls the grade down sharply.

A ring is worn on a hand that knocks into things, so ask these questions before you commit to a stone:

• Does any feather or crystal reach the surface?
• Is it near the edge, where prongs will grip, or will a bezel cover that part of the girdle?
• Does the plotted diagram on the report match what you see through a loupe?

Choose the stone first, then the setting that best protects its weak spots.

Grades are set at 10x, but a ring is worn at arm's length. The International Gem Society describes the SI grades as having inclusions that are noticeable at 10x but most likely not to the naked eye. GIA reminds buyers that two diamonds with identical reports will still look different, so a grade is a starting point rather than a verdict.

Read the plot on the diamond grading report to see what each feature is and where it sits, then look at the stone face-up and from the side. A feature under the table is easy to see. One near the edge may be hidden by a prong. Ask, too, whether the stone has been treated. GIA's grading process starts by addressing any clarity enhancement or surface coating, and diamond treatments can change how a feather looks.

To talk a stone through, visit a showroom, browse engagement rings, or plan a custom engagement ring.