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Chapter 10 - Gems From Metamorphic Rocks

Once any form of rock has formed (igneous, sedimentary, or metamorphic) it can be altered to another form (process is called metamorphism). The process can be rapid (such as by an asteroid impact, or by the sudden changes in the subsurface environment by an igneous eruption or massive earthquake). The introduction of hot fluids (water and gases) also can relatively quickly alter rocks into different materials. However, on a large scale, most metamorphism take place slowly as materials, once on or near the earth's surface, are subjected to increasing temperature and pressure conditions with increasing burial. They can also be altered as they rise back toward surface condition (retrograde metamorphism). Most gem varieties of metamorphic origin are highly durable (having high tenacity) that allow them to survive the long journey back to the surface to be exposed to weathering and erosion. This chapter looks at many of the common gem minerals that form in metamorphic environments. Most of these physically- and chemically-active environmental settings occur in association with converging tectonic plates (Figures 10-1 to 10-6).
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Metamorphic facies
Fig. 10-1. Metamorphic facies in Plate Tectonics
Examples of rocks from the 5 major metamorphic facies (from the previous chapter). The plate-tectonics figure shows the geologic setting in which most rocks of these different groups form.
Blue and green schist Blueschist Amphibolite schist Eclogite Granulite
Fig. 10-2. Greenschist Fig. 10-3. Blueschist Fig. 10-4. Amphibolite Fig. 10-5. Eclogite Fig. 10-6. Granulite

Gems from metamorphic rocks

Gems from metamorphic rocks include popular faceted crystal varieties: corundum (including ruby and sapphires), beryls (including emeralds), garnet, spinel, and zircon. Non-faceted gem minerals (one that are usually cut and polished into cabachons or shapes) include jade (jadeite),and quartz varieties (jasper, carnelian and tiger eye), and copper-based gemstones including lapis lazuli and turquoise. Other metamorphic minerals used as gemstones include andalucite, epidote, enstatite, kyanite, and staurolite. Although gem-quality diamond also form under the extreme heat and pressure conditions in the upper mantle depths, they only find their way to the surface in unusual igneous processes (associated with kimberlites and related igneous rocks).

Corundum varieties include Ruby and Sapphire.
Corundum is a very hard mineral of aluminum oxide (Al2O3) that occurs in fine- to coarse-grained massive and crystalline forms. Gemstone varieties are ruby (red) and sapphires (commonly blue, but include yellow and green varieties)(Figures 10-7 and 10-8).

Beryl
includes gem varieties include emerald, aquamarine, heliodor, and morganite, however only some of these form in metamorphic setting (Figure 10-9). Emeralds are a classic example of a gemstone usually found in metamorphosed shales (due to regional metamorphism) or quite separately in veins of hydrothermal origin (in association with contact metamorphism)(Figure 10-10). Because metamorphism takes place in a solid state, inclusions of other solids are usually trapped as the emerald grows around them. Emerald having inclusions of other minerals is so typical, that any flawless emerald is immediately suspected as a synthetic or even a fake! The inclusions in an emerald are referred to as its "jardin" (French for Garden).

Spinel (Figure 10-11)

Zircon
(Figure 10-12)

Garnet is a common mineral in moderate- to high-grade metamorphic rocks, with pyrote and almandine being the most common amongst more than 30 mineral varieties (Figures 10-13 and 10-14). Many garnets form in metamorphic rocks that are formed in geologic environments where pressure is the predominant agent of metamorphisis (some garnets do form in igneous rocks). Where as garnets are common in many places, because they form under extreme pressure conditions, most common garnet crystals appear highly fractured when exposed to surface conditions.

Jade is the name commonly given to two different metamorphic mineral, nephrite (a green, fairly soft fiberous mineral) and a much harder, a variety more used as gemstone, jadeite. The mineral jadeite develops when igneous rocks in oceanic crust are “pinched” under high pressure. In some locations, this old ocean crust has been pushed up onto the continent by these pressures (Figure 10-15). The rock was then changed from basalt and gabbro to metamorphic rock commonly called serpentine. (More accurately, the rock serpentinite consists dominantly of variety of serpentine minerals of which there are many varieties). Serpentinite is exposed throughout much of the Coastal Ranges of northern California. It is also exposed in small patches of serpentine are also found on Staten Island and Manhattan and suggest that ocean crust was pushed onto land in an ancient mountain belt making an “Ophiolite.” The mineral graphite is formed typically from organic sedimentary rocks rich in carbon such as coal beds that have been metamorphosed.

Quartz includes metamorphic varieties carnelian, tigers eye, and jasper) (Figures 10-16 to 10-18). Although quartz is abundant in metamorphic rocks it is typically milky white and masses do not form crystal forms as occur in sedimentary and igneous settings where well developed crystals can grow without confinement. Jasper is a typically red variety of chert (or metachert) that has a red color, but chert can occur in practically any color of the rainbow, including black and white, and particularly in metamorphic settings the colors can be mixed and preserve high fractured, cortorted, or orbicular textures. "Orbicular" means having a ring-like, rounded-convex or globular appearance

Chert is a sedimentary rocks that when subjected to metamorphism becomes "metachert." It can be extremely hard and is a favorite of collectors because it is both abundant and can be quite colorful, good for cutting and polishing into cabechon-style jewelry. Figure 10-16 is an example of "poppy jasper" a popular variety of metachert from the central California region that displays an orbicular color texture.

Lapis Lazuli (Figure 10-17)

Turquoise (Figure 10-18)
Ruby in Varisite sapphire cut gem
Fig. 10-7. Ruby (red hexagonal crystal) in variscite (green mineral). Fig. 10-8. Sapphire (typically blue, but varieties may green to yellow in color).
   
Fig. 10-9. Beryl Fig. 10-10. Emerald
   
Fig. 10-11. Spinel Fig. 10-12. Zircon
Gore Mountain garnet Garnet crystal from Garnet Hill, Calaveras County, CA
Fig. 10-13. Almandine garnet (red) from Gore Mountain, New York Fig. 10-14. Andradite garnet from Garnet Hill, Calaveras County, CA
Jade from Jade Beach, California Charmeleon
Fig. 10-15. Jade from Jade Beach, Big Sur, California Fig. 10-16. Carnelian
Tiger Eye poppy jasper
Fig. 10-17. Tiger's Eye Fig. 10-18. Poppy jaspser (a variety of metachert).
   
Fig. 10-19. Lapis lazuli Fig. 10-20. Turquoise
   
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9/17/2014