Prof. Robert Stern

Professor Robert Stern provided the chapter members with a a very informative discussion of how gemstones form in a variety of geologic settings. Plate Tectonic Gemstones – Geology 2013

Almost all gems of mineral origin form in the Earth’s crust, with the notable exceptions of peridot and diamond, which form in the mantle, and all of them are mined in or on the Earth’s crust. This gemiferous crust is made up of three types of rocks: igneous, sedimentary and metamorphic, which differ in their origin and characteristics.

Igneous rocks are those which solidify from a molten state, sedimentary rocks form due to consolidation of layered sediments, evaporates, or precipitates, and finally metamorphic rocks result when great temperature and pressure change the crystal structure of either igneous, sedimentary, or other metamorphic rocks.

As you can expect many things happen deep in the Earth but certain gemstones are only found in certain environments and can be used to identify the Plate Tectonic environments in which they form. Understanding these tectonic environments helps gemologists understand these gemstones and the location of diagnostic gemstones helps geoscientists understand Plate Tectonics.

The talk focused on three (3) “Plate Tectonic Gemstones”: Jadeite, Ruby, and Diamond.

Jadeitte is the “Subduction Gemstone”
Jadeite forms in the subduction zones, where the seafloor and the underlying oceanic
crust along with the upper mantle sink back into the deeper mantle. Jade forms when
fluids released from subducting oceanic crust condense in the mantle wedge 20-120 KM deep in the earth.

Ruby is the “Collision Gemstone”
Ruby forms where continents collided. Rubies are a type of corundum, a rare mineral made up of densely packed aluminum and oxygen atoms, which are normally colorless, but when other atoms are substituted for a few of the aluminum ones, bright hues emerge. In Ruby small amounts of chromium impart the deep red color.

Ruby is formed by undergoing transformation by heat and pressure in the folding of strata with the nearby igneous rock. Igneous rocks form when magma (molten rock) rises from deep underground and cools and solidifies at or near the Earth’s surface. Igneous rock that forms under the ground may later reach the surface because of geological upheaval or when the surface wears away. Metamorphic rocks, like Ruby, are formed when existing
rocks are changed underground by great heat or pressure, or both. When volcanoes erupt and when mountains are formed by the movement of the Earth’s tectonic plates, rocks are heated and squeezed. The minerals in the rocks are then changed, forming metamorphic Rubies 5-35 KM deep in the earth.

Diamonds are not formed by Plate Tectonics, but are the result of explosive eruption of
kimberlite that carries diamond to the surface as part of the delivery of large volumes of water and carbonate deep in the mantle by subduction zones.

The scenario goes something like this:
Magma, containing diamond crystals, suddenly and explosively finds a path to the surface.

As the lava rises, some of it cools and solidifies underground forming a carrot shaped formation of kimberlite rock, in which the diamond crystals are “frozen”.

The volcanic cone erodes away leaving diamonds at the surface, and underground in the kimberlite (or lamproite) “pipe”.

Summary by Charlie Marts