Ruby is corundum, the same mineral as sapphire, coloured red by chromium. That chromium is also what makes fine ruby fluoresce, giving the best stones an internal glow that no photograph captures properly. It ranks nine on the hardness scale, so like sapphire it stands up to daily wear.This collection includes the 0.85ct Ruby Devil’s Heart Pendant and the 2.50ct Ruby Theo Fennell Heart Pendant, both pieces where the stone carries the design, alongside Evelyn and Kyoto.Burmese ruby, particularly the colour known in the trade as pigeon’s blood, sets the standard and prices accordingly. Mozambique has become a major source of excellent material at more accessible levels. Heat treatment is routine and disclosed. Glass filled rubies are a different matter entirely, sold cheaply and worth very little, and knowing the difference before you buy is the whole point of dealing with a jeweller who will tell you.Fine ruby is rarer than comparable diamond and, at the top of the market, more expensive per carat. It is the July birthstone and the fortieth anniversary stone. Our buyer guide to rubies explains origin, treatment and how to avoid the filled material, and gemstone guides covers the coloured stones together. If you have a ruby you are thinking of selling, we also buy loose gems and diamonds.