RB1 is the highest tier of Lotus Gemology's royal blue colour scale for sapphire. The designation is reserved for stones that match the laboratory's tightest reference window for the classic royal blue description: a vivid, medium-strongly saturated blue of medium tone, with a slight violet bias and no green modifier, free of grey, and showing the blue clean across daylight and tungsten illumination.

The reference window

The Lotus royal blue scale is calibrated against published reference comparators in the same manner as the pigeon blood scale for ruby, with RB1, RB2 and RB3 forming descending tiers within the royal blue category. RB1 is the apex - the stones whose colour is closest to the historical Burmese and Kashmiri exemplars of the term as Lotus has defined it.

Origin context

RB1 stones are most often Burmese (Mogok and, less commonly, Mong Hsu) or Kashmiri, but the colour designation is independent of origin: a Sri Lankan or Madagascan sapphire that meets the colour window will earn an RB1 call. As with PB1 in ruby, the highest-priced RB1 stones in the market combine the colour designation, an unheated treatment status, and a top-tier origin determination, where the three together produce stones in the most exclusive price band of the sapphire market.

Use in the trade

For the working dealer, an RB1 designation flags a stone at the apex of the laboratory's royal blue grade. RB1 stones trade at substantial premiums to RB2 and RB3 stones, with the differentials widening at the larger sizes where RB1-eligible material becomes vanishingly rare. As with all Lotus colour grades, the call should be read alongside the laboratory's complete report, particularly the heat-treatment status and the origin determination.