Where do Vietnamese rubies originate from?

Vietnamese rubies primarily originate from deposits that are found in metamorphic rocks. Among the choices provided, marble is a type of metamorphic rock that can be associated with the presence of rubies, particularly in regions where high-grade metamorphic processes have occurred.

In the case of Vietnamese rubies, the primary source is often associated with geological formations that involve the metamorphosis of limestone into marble. This can create the right conditions for the crystallization and formation of rubies within the metamorphic environment.

Other rocks listed, such as limestone and granite, can contain minerals and gemstones, but they do not typically result in the formation of rubies in the same way that marble can. Basalt, being an igneous rock, is also less likely to be the source of rubies due to its different formation process. Thus, considering the geological context and formation processes, marble best represents the mineralogical environment conducive to the occurrence of Vietnamese rubies.