Forsterite is only associated with blue spinel, and clinohumite is only associated with red spinel. Clinohumite can also grow from diopside in a prograde reaction with dolomite and water (Proyer et al., 2008). Ruby-bearing rocks are very different from those containing blue spinel, as they underwent different metamorphic histories. Because of the intense tectonic activity in Luc Yen, it is possible that two rocks with very different geological histories have been brought in contact.