Believe it or not, most diamonds have birthmarks. Called “inclusions,” they determine how clear, or pure, a diamond is. Just remember this little rhyme: Clarity is a rarity.

Inclusions can’t usually be seen without magnification. (This is one reason jewelers use loupes.) The fewer and smaller the inclusions, the clearer, rarer and more beautiful the diamond.

The scale for grading clarity ranges from “internally flawless” (IF) to “imperfect” (I1, I2 and I3). The naked eye can usually spot inclusions in the “imperfect” grouping. After IF, in order of decreasing clarity, there’s “very, very small” (VVS1, VVS2), “very small inclusions” (VS1, VS2) and “small inclusions” (SI1, SI2, SI3).

Below, you can hear Ronnie Mervis explain clarity.

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