In some cases, nephrite jade may also form from the metasomatic replacement of dolomite by silica-rich aqueous fluids during the emplacement of felsic plutons ( Harlow and Sorensen, 2005). Both types of jade originate at convergent-margin tectonic settings either by the replacement of serpentinite at low-pressure and low- to moderate-temperature conditions or by the metasomatism of peridotite at high-pressure and low-temperature conditions. Most jadeite and nephrite jade deposits owe their origins to fluid interaction with their host serpentinite, a metamorphic rock mostly composed of serpentine group minerals (e.g., antigorite, lizardite, and chrysotile) produced by the hydrous alteration of ultramafic rocks. Metamorphic environments characterized by high temperature and high pressure can produce jade or in rare cases diamonds. Gem-quality spinel occurs in marbles and calc-silicate rocks that have undergone high-grade (high temperature) metamorphism and in skarns formed at the contact zones between calcium-rich rocks and magmatic intrusions. Corundum of metamorphic origin are formed in marble (e.g., the ruby deposits of Mogok, Myanmar), or by fluid-rock interaction (metasomatism) developed at the contact of desilicated pegmatites that have intruded mafic and ultramafic rocks (e.g., the sapphire deposits of Kashmir, India), skarn deposits, and shear zoneвЂ“related deposits, represent metamorphic rocks that host gem corundum. Gem corundum can form naturally by the metamorphism of alumina-rich rocks under high temperature conditions. Typical gem minerals of metamorphic origin include almandine and pyrope garnet, andalusite, kyanite, and iolite, the gem variety of cordierite. The type of gem mineral occurring in metamorphic rocks depends on the conditions of metamorphism and the composition of the original rock prior to metamorphism. Common metamorphic rocks that may host gem minerals include schist, gneiss and marble. Metamorphic rocks are igneous, sedimentary, or metamorphic rocks that have been changed or altered in response to deep burial, intense heat and pressure without melting the rock or interaction with hot fluids. Michael Wise, in Encyclopedia of Geology (Second Edition), 2021 Gem Minerals From Metamorphic Rocks Metamorphic rocks are exceptionally appreciated as decorative and building stone due to crystalline texture, layering, brilliant colors, and excellent polishing capabilities. Plutonic metamorphism occurs at high temperatures and strong pressure in deeper parts of the lithosphere producing granulites, eclogites, and migmatites. Regional metamorphism, under extreme high temperature and uniform pressure over large areas of continental crust forms high-grade metamorphic rocks like slate, phyllite, amphibolites, varieties of schists, para- and ortho-gneisses, quartzite, and marble.

Contact metamorphism by the thermal effect of magma/lava generates skarns. Dynamic metamorphism is due to mechanical deformations during tectonic movements forming mylonites, flazer, and augen gneisses. Types of metamorphism include dynamic/kinetic, contact, regional, and plutonic. Structures include gneissic, schistose, and slaty. Common textures are crystalline, granular, xenoblasts, idioblasts, granoblastic, and porphyrpblastic. The pressure is caused by the weight of sediments or crust. The sources of temperature are geothermic gradient, effect of magmatic body, and friction in rock masses of tectonic movements following prograde or retrograde mechanism. Metamorphism alters the mineral composition including the formation of new minerals (garnet, zoisite, kyanite, chlorite, biotite, sericite, staurolite, sillimanite, talc, and andalusite). Metamorphic rocks are the product of transformation or solid-state recrystallization of existing (protolith) igneous, sedimentary, and metamorphic rocks, by the change in physical and chemical conditions, principally temperature, pressure, and introduction of chemically active fluids. Haldar, in Introduction to Mineralogy and Petrology (Second Edition), 2020 Abstract

Marble index minerals ternary diagram