Ruby

Ruby

The red variety of corundum, coloured by chromium and valued above all for the purity and intensity of its hue.

Overview

Ruby is the red variety of the mineral corundum, an aluminium oxide. A small amount of chromium replacing aluminium in the crystal produces both the red body colour and, in low-iron stones, the red fluorescence that makes fine ruby appear to glow in daylight.

Rubies form in two broad geological settings. Marble-hosted deposits such as those of Myanmar yield stones low in iron with strong fluorescence, while basalt-related and amphibole-related deposits such as those of Thailand and Mozambique tend to be richer in iron and darker in tone.

Heat treatment is common and long established, and some material is also fracture-filled with glass. Much of our ruby work therefore concentrates on documenting inclusions and trace-element chemistry that indicate both geographic origin and treatment history.