Why should you know what fire is? It is one of the things your eye notices first when you look at a diamond. Fire has traditionally been one of the key elements in judging a diamond’s overall appearance 1. GIA counts fire as one of the three appearance-based parts of its cut grade. The other two are brightness and scintillation, which is sparkle 4.
How does a diamond make colour from white light? Fire is the visible result of dispersion, the splitting of white light into its separate colours. When light enters a polished diamond, it bends and starts to separate. The first spread is very slight, but it widens with each facet (flat polished surface) the light meets 1. This is why the shape and angles of the facets matter. The lengths of some facets can noticeably affect fire 2.
What changes how much fire you see? Lighting plays a big part. A single spotlight brings out fire, while diffuse light, such as a fluorescent desk lamp, suppresses it 3. More fire is also not the same as diamond. Synthetic moissanite has a dispersion of 0.104, more than twice diamond’s 0.044 5. For how these stones compare, see Natural Diamond, Laboratory-Grown Diamond or Moissanite?.
Example: Under a small ceiling spotlight, you tilt your hand and your diamond flashes orange, green and blue. Back at your desk under a fluorescent lamp, the same diamond shows mostly white light, because diffuse lighting hides fire 3.