Where do Gemstones Come From? Extra Notes

Rose Quartz Mine, Cabo Delgado, Mozambique.

Terms:

Attached- Attached crystals are those which grow ‘attached’ to another rock an hence can only have one termination etc. Eg. Diopside, pyrite, quartz. In-situ- In the place of formation, e.g. emerald and tourmaline are often found in-situ. Disseminated- Disseminated crystals grow dispersed within other rocks and are commonly anhedral due to growth restriction. Eg. diamond, garnet, ruby. Gem gravel- Gem occurrences in gravels, clays, and other loose deposits derived by weathering of earlier rocks. Eg. sapphire, spinel, zircon. Vugh- Another name for geodes. Note that vugs (also called vughs) are small cavities inside rock that are formed when crystals form inside a rock matrix and are later removed through erosive processes, leaving behind voids. Also can be defined as ‘a cavity in a lode’. Geode- Cavities produced by gas bubbles in a molten rock mass and present when the rock solidifies may be invaded by hydrothermal solutions, producing geodes. In the gods the crystals radiate inwards from the walls forming mineral specimens. Pegmatite- Course grained, quartz felspar rocks formed by the consolidation of the last portion of magma. Usually forms as vein structures in the surrounding rocks. Eg. beryl, topaz tourmaline. Primary Rocks- Original rocks formed through a geological process not altered in any way. Secondary deposit- Deposits formed by very late stage magmatic solutions or by circulating groundwater. Eg. Opal, quartz. Metasomatism- The processes whereby the chemical composition of a rock is altered by the addition or removal of material by solution in fluids/volatiles and hence new mineral assemblages are formed. Commonly in association with contact metamorphism  

Igneous

Rocks formed by the solidification of molten rock (magma/lava) within or on the surface of the earth. Igneous rocks can be classified into plutonic, formed deep within the earth, hypabyssal, formed at intermediate depths and volcanic formed on the surface. These can be further classified into felsic, intermediate or mafic according to composition.   Major Mineral Associations Quartz Orthoclase feldspar Plagioclase feldspar Ferromagnesian minerals Etc. Major Gem Associations Diamonds Obsidian Corundum Beryl Zircon Spinel Olivine Etc.

Metamorphic:

Rocks formed through the alteration in the solid state of existing rocks by high temperatures and/or pressure. There are two main types of metamorphism, these are regional, which is extensive metamorphism of the crust as a result of the stresses and high temperatures associated with plate convergence, collision and subduction and commonly has associated foliation. The second is contact metamorphism which occurs adjacent to bodies of hot magma that have intruded into cooler rocks. Contact metamorphism involves mainly chemical recrystallization in response to a pronounced increase in temperature and to the involvement of chemically active fluids released by the magma. Major Mineral Associations Micas Serpentinites Etc. Major Gem Associations Iolite Zoisite Epidote Garnet Diopside Kyanite Etc.

Sedimentary:

Rocks composed of clastic sediments formed of fragments of other rocks transported from their sources by wind water or ice etc. and lithofied after deposition. Can also be non-clastic sediments formed by chemical precipitation from solution or by biological processes. Major Mineral Associations Carbonates Platey minerals (clay minerals etc.) Clastic sediments Etc. Major Gem Associations Opal Rhodochrosite Calcite Rhodonite Tiger eye Amethyst Malachite Etc.