Encyclopedia of geology, five volume set, volume 1 5 (encyclopedia of geology series) ( PDFDrive ) 1195
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GEMSTONES 13
of the mineral actinolite and shiny yellowish-brown
mica flakes. Indian emeralds have a characteristic
two-phase inclusion that resembles a comma with a
jagged outline.
scale with mechanized vehicles and diggers. In order
to escape the extreme heat, some miners live in
subterranean houses, where only the rooftops show
above the surface.
Other Sedimentary
Gemstone Deposits
Conclusion
In addition to alluvial and eluvial gemstone deposits, gemstones may also form in veins or cavities
within rocks or as crusts on their surfaces. Following
transportation, they react with other elements to
produce new minerals or may be deposited directly
from solutions. New minerals may also form deposits when water cools, evaporates, or changes level,
leaving a mineral residue. Examples of gemstones
formed in rocks as a result of the evaporation, cooling, or transportation of mineral-rich fluids include
turquoise, malachite, rhodochrosite, agate, and
amethyst.
Turquoise usually forms as a thin layer or vein
within the rock. The rock is mined and the turquoise
removed for polishing or fashioning, often as beads or
small pieces for use as inlays. Malachite can occur as
large botryoidal (rounded) masses or as thin layers
and is often associated with azurite. Rhodochrosite
can occur as large outcrops, which are mined using
heavy machinery and cut and polished as smaller
decorative pieces.
Agates are fairly common and are usually found as
harder geodes within rock faces of igneous origin.
Underground mining, following rock layers containing the geodes, results in a labyrinth of tunnels
and mineshafts. The agate mines in Idar-Oberstein
(Germany), which have been mined for agates since
Roman times, are now popular with tourists and
groups of visiting gemmologists.
Opal is formed in sedimentary rock where organic
remains have decomposed to leave minerals within
the rock or cavities into which minerals have been
transported by fluids. Large specimens, such as fossilized dinosaur bones, may be replaced by opal, and
small irregular voids within a rock can be filled.
Australia is the best-known and most important
source of opal. The main mining areas are Lightning
Ridge, Coober Pedy, White Cliffs, Quilpie, and
Andamooka. Tourists can obtain licenses to mine,
checking the ground and riverbeds for small rock
fragments containing opal and panning rivers during
their stay. Mining companies work on a larger
The mining method used depends on whether the
gemstones are found in situ (in the host rock in
which they formed) or as secondary placer deposits
eroded from or washed out of the host rock and
accumulated elsewhere. The locality, its accessibility,
the availability of a local workforce, and the political
situation are also factors. The specific properties of
gemstones – their beauty, rarity, durability, and value,
which is often a result of fashion as much as size or
clarity – will also affect the mining method used. The
method chosen should not damage fragile or particularly valuable specimens, as the aim is to maximize
production and, ultimately, profit.
See Also
Building Stone. Fluid Inclusions. Igneous Rocks: Kimberlite. Mining Geology: Exploration; Mineral Reserves;
Hydrothermal Ores. Sedimentary Environments: Alluvial Fans, Alluvial Sediments and Settings. Weathering.
Further Reading
Hall C (1993) Gems and Precious Gemstones. London:
Apple Press.
Hall C (1994) DK Eyewitness Handbook Gemstones, 2nd
edn. London: Dorling Kindersley.
Hughes RW (1997) Rubies and Sapphires. Boulder:
R W H Publishing.
Mumme I (1988) The World of Sapphire. Port Hacking:
Mumme Publications.
Oldershaw C (2003) Philip’s Guide to Gems. London:
Philips.
Oldershaw C, Woodward C, and Harding R (2001)
Gemstones, 2nd edn. London: Natural History Museum
Publications.
Sevdermish M and Mashiah A (1996) The Dealer’s Book
of Gems and Diamonds, 2nd edn. Gemmology (A M)
Publishers Ltd.
Sinkankhas J (1981) Emeralds and Other Beryls. Radnor,
PA: Chilton Press.
Webster R (1994) Read PG (ed.) Gems: Their Sources,
Descriptions and Identification, 5th revised edn. London:
Butterworths.
