Gems Found in Sedimentary Rocks
Gems & Gemology
James E. Shigley, Roy Bassoo, and Aaron C. Palke
Sedimentary rocks such as shale, limestone, and sandstone are among the most common rocks exposed on the earth’s surface (figure 1). In actuality they represent only a thin covering of the crust, which mainly consists of igneous and metamorphic rocks. Despite their relatively insignificant volume, many of the major geologic events in the earth’s history are more accurately documented and age-dated by their scientific study.
In addition to having economic deposits of oil, natural gas, coal, and certain heavy minerals such as gold and platinum, sedimentary rocks also host important primary and secondary occurrences of many gemstones (Dill, 2018). The occurrences of gems in sedimentary rocks result from two different geological processes. Opal and turquoise, for example, can form in voids or empty spaces by precipitation from water or mineralized solutions, respectively (figure 2).
In contrast, other gems such as diamond and sapphire were weathered out of their original igneous or metamorphic host rocks and then carried away by erosion to be later trapped in sediments. Their hardness and resistance to mechanical abrasion allowed these minerals to be transported and then deposited by moving water to create alluvial or placer gem deposits (figures 3 and 4). Both categories of gems in sediments will be discussed in this column.
Editors’ note: Questions or topics of interest should be directed to Aaron Palke ([email protected]) or James Shigley ([email protected]).
GEMS & GEMOLOGY, VOL. 59, NO. 4, pp. 510–523. © 2023 Gemological Institute of America
Sedimentary Rock Formation
A three-step sequence of geologic processes results in sedimentary rocks forming on or near the earth’s surface. The first step is termed weathering—the physical, chemical, and biological decomposition of preexisting rocks of all kinds to form finer-sized particles. These particles accumulate and compact in geological basins over time to form new sedimentary rocks. Weathering involves the combined actions of air, water, chemical salts and/or solutions, freezing and thawing temperature cycles, and the effects of plants and animals on rocks.
The change in relative abundance of the various minerals from the original rock to the final sedimentary products is a measure of their comparative resistance to weathering. In our Winter 2022 column, titled “Gems Formed in Magmatic Rocks,” we discussed the concept of “Bowen’s reaction series” (figure 7 in Palke and Shigley, 2022).
Based on field observations of rock relationships and experimental studies of their formation, this concept describes the sequence of formation of the common rock-forming silicate minerals at specific decreasing temperatures from a cooling magma. In a similar way, geologists have recognized a “mineral dissolution series” of the relative stability (or “resistance to weathering”) of common minerals at the earth’s surface (figure 5). This dissolution series is also based on several other factors, including the extent of bonding of silica tetrahedra in the mineral’s crystal structures, as well as the relative resistance of their chemical elements to leaching by water. Minerals that are more resistant to weathering and dissolution are more likely to be retained in sedimentary environments and concentrated in alluvial deposits.
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