Beyond the 4Cs
Millions of years ago, when diamonds were forming as crystals of pure carbon in the Earth's mantle, minute traces of other minerals or gas were sometimes trapped inside the matrix. These inclusions may appear as tiny specks, lines, feathers or clouds inside a diamond. They are nature's fingerprints. Fewer and less visible fingerprints make a diamond more valuable.
Clarity is described using an international grading scale – with the most valuable and rare designation being flawless (FL-IF with no flaws visible under 10X binocular microscope), and the least valuable being heavily included (I3, with flaws easily visible to the naked eye).
Hearts On Fire diamonds are strictly limited to the FL (flawless) to the SI2 (slight inclusion) range.