This describes the different kinds of rocks where gemstones are found.
By their very nature it is virtually impossible to identify a specific “Gem Ore” natural gems are found in all sorts of rocks varying from igneous, metamorphic and sedimentary. It all depends on what sort of gems you are looking for. Two rock types are known to hold diamonds and one is known to hold beryl and topaz gemstones. Most gems coming to the market place are gemstones that have been eroded from the rock containing them and are found in alluvial gravel deposits. This is the closest thing there is to a gem ore there is on Earth.
Diamonds are found in two intrusive igneous rocks kimberlite and lamproite. Both of these rocks come from depths ranging from 100 to 150 km in the mantle of the earth where as far as we can tell diamonds are a common rock forming material. When these intrusions of ultramafic magma reach the surface they cause a very violent eruption that hasn’t been seen in the past 60 million years, fortunately. The same intrusions also contain some other minerals that are considered gemstones chrome diopside and pyrope garnet. Diamonds are also found in alluvial deposits in rivers and along beaches in diamond bearing country.
The other type of igneous rock where gemstones is found is a pegmatite that is chemically the same as granite except because pegmatites have cooled slowly the crystals produced are gigantic. Pegmatites are mainly composed of feldspar, quartz and mica. They also contain more then 100 other minerals many of which are identified as gems including beryl, topaz and tourmaline.
Metamorphic rocks contain a totally different suite of gemstones starting with the rarest of all, the ruby. They are also capable of containing other members of the sapphire family besides rubies. These gemstones are all composed of aluminum oxide that in its natural form is called “corundum.” The play of colors found in this species determines its color, and often its crystal habit is the result of trace elements the most common of which are iron and chrome.
This family of gemstones is a result of regional metamorphism of a “dirty limestone” that has been contaminated with clay before it is metamorphosed. The process of meta-morphism turns the entrained clay into corundum crystals.
Emeralds are another gemstone that is also found in metamorphic rocks; in this case in rocks that have undergone contact metamorphism around an intrusion of molten magma. The emerald deposits found in Columbia are of this type. If carefully examined virtually all emeralds are found in this environment. Usually emeralds are found as minerals in contact metamorphic aureoles around pegmatites and are associated with pyrite in a black colored fetid dolomite.
An entirely different suite of gemstones are found in sedimentary rocks especially in conglomerates that contain gemstones including diamonds that are normally found in alluvial gravel deposits. Conglomerate is a fossilized gravel deposit.
Many of the cryptocrystalline silica such as agates and jasper are found in sedimentary rocks. Going up the scale precious opals are found in sandstone in places like Australia, Mexico and in the United States including Virgin Valley, Nevada.
In most cases these cryptocrystalline silica gemstones are secondary deposits from geothermal waters rising upwards through the rock from the depths of the earth.
There are many places on Earth that produce gemstones with one of the most important recently found in Afghanistan, but the list of countries goes all the way to Zaire, so take your choice.
Geology of Gem Deposits, ed. Lee A. Groat, Springer Verlag, 2008, http://www.springerlink.com/content/x3808h2267r56w54/
Smithsonian Gem & Mineral Collection, http://www.gimizu.de/sgmcol/