These are diamonds that have a medium, strong or even very strong blue fluorescence. Actually Brian Gavin did not sell diamonds with blue fluorescence since 1998. Brian Gavin noticed that many customers returned diamonds with blue fluorescence. This was in the beginning of e-commerce. People who bought their diamonds online often had their diamonds checked by brick and mortar store owners. These naturally told the clients that diamonds with blue fluorescence are undesirable and should be returned!