The Mogok metamorphic rocks of the Mogok Metamorphic Belt (MMB) host some of the world’s finest rubies, mainly from high-grade phlogopite+ corundum marbles and sapphires from a heterogeneous suite of syenite and charnockites that intrude the metamorphic rocks. Gem-qyality spinels are also found in forsterite-bearing marbles.

Metamorphic spinels are mainly found in these suits or analogies, such as mafic (Fe,Mg-rich rocks) intruded into silica undersaturated rocks (like marble, syenite above). If you put too much SiO2, you will lose valuable stones but less-valued kyanite, silimanite and andalusite lol (also interesting in geology). Pure spinel should be colorless, however, trace chromium or cobalt can make a huge change to form neon red to blue color which are "hot topics" in the precious gem market.

This is a good example I purchased several years ago that shows the twined spinel paragenesis of phlogopite with rutile (and garnet?) inclusions as an indicator of the forming environment even without seeing the marble matrix.

1.5cm, Mogok Township, Pyin-Oo-Lwin District, Mandalay Region, Myanmar

Jingnan (Tom) Zhang Coll and Photo