Next to red diamonds, rubies are probably the most expensive of gems in the world in sizes over 3 ct. The ideal for ruby is that of a red traffic signal, a highly fluorescent red of high intensity. Unlike diamond, small amounts of silk in a stone actually help the colour because they scatter light into areas it would not otherwise go. This helps cover up the extinction which would otherwise be found. Thai/Cambodian rubies suffer the double deficiency of too much iron, which cuts the fluorescence, and no silk to scatter light.