H Samuel a chain store in the UK owned by Signet group have now got an ideal cut hearts and arrows diamond called Celebration Grand cut in Tolkowsky style. They are saying this makes a 1 carat have a larger diameter of 6.4mm compared to the 1 carat standard of 6mm. I thought when I learned disamonds on price scope and the internet that 6.5mm was standard so is that only in the USA that this standard is held?