Sedimentary processes shape gems in two ways. Some gems grow in the cool, wet conditions near Earth's surface, such as opal, turquoise, malachite and chalcedony. Others formed deep underground but were freed by weathering and concentrated by rivers and waves into gravel deposits, which is where much of the world's sapphire, spinel and diamond has been found. Amber, preserved in sedimentary layers, adds an organic gem to the list.
Weathering, erosion and sediment
Weathering breaks rock down where it sits. Physical weathering cracks rock through freezing, heating and root growth. Chemical weathering dissolves minerals or turns them into clay, and it works fastest in warm, wet climates. Erosion then moves the loose material by water, wind, ice and gravity.
Sediment builds up in low places such as river valleys, lakes, deltas and the sea floor. Over time it is buried, squeezed and cemented by minerals deposited from groundwater, becoming sedimentary rock such as sandstone, shale or limestone. Sedimentary rocks cover most of the land surface, though they make up only a thin layer compared with the igneous and metamorphic rocks below.
Gems react differently to weathering. Hard, chemically stable minerals such as corundum, spinel, zircon, topaz, garnet, chrysoberyl and diamond survive almost unchanged. Softer or less stable minerals break down into clay or dissolve. That difference is what makes gem gravels possible.
Opal in sedimentary basins
Australia's precious opal fields lie in sedimentary rocks of the Great Artesian Basin, which was covered by an inland sea during the Cretaceous period. As the region weathered deeply, silica was released from minerals in the rock and dissolved in groundwater. The water moved down through the rock and pooled in cracks, cavities and around fossils. As it slowly evaporated or the chemistry changed, the silica settled out as tiny spheres. Where those spheres were all the same size and packed in a regular array, they formed precious opal with play-of-color.
This explains some well-known features of Australian opal. Lightning Ridge black opal comes from nodules called nobbies in claystone. Coober Pedy and Andamooka opal fills cracks in weathered sandstone and claystone. Boulder opal fills gaps in ironstone concretions in Queensland. Opalized fossils, including shells and bones, form where silica replaced the original material.
Ethiopian opal from the Wollo area is different. It occurs in layers of altered volcanic rock, so it is not a sedimentary deposit in the strict sense, even though it also formed from low-temperature silica-rich water.
Alluvial gem gravels
Rivers sort sediment by size and density. Heavy minerals drop out where the current slows, such as on the inside of bends, behind boulders, in potholes and at the base of gravel layers. Gemstones are denser than quartz sand, so they collect in these spots along with other heavy minerals. Geologists call these accumulations placer deposits.
Alluvial gravels have some advantages for miners. The gems are already freed from hard rock, so they can be dug and washed with simple tools. River transport also breaks weaker crystals along cracks, so the stones that survive tend to be cleaner. Waterworn gems often have rounded, frosted surfaces.
Sri Lanka's illam
Sri Lanka is the classic example. Around Ratnapura and other districts, gems eroded from old metamorphic rocks and settled in a gravel layer called illam, buried under clay and silt. Miners dig pits or short shafts down to the illam, haul the gravel up in baskets and wash it in round baskets in streams. The gravel yields sapphire, padparadscha, spinel, chrysoberyl and cat's eye, garnet, zircon, topaz and moonstone. The gems of Sri Lanka page describes these fields.
Other gem gravels
Myanmar's Mogok valley produces much of its ruby and spinel from gravels called byon. Madagascar's Ilakaka sapphire, the Chanthaburi sapphires of Thailand and the sapphire fields of eastern Australia are mined largely from alluvial gravels and weathered soil. In Brazil, gem gravels yield diamonds, topaz and chrysoberyl.
Placer diamonds
Diamond is both hard and dense, so it collects well in placers. Until the discovery of kimberlite pipes in South Africa in the 1870s, all diamonds came from alluvial deposits, first in India's Krishna River valley around Golconda and later in Brazil. Diamonds carried down the Orange River reached the Atlantic and were spread along the coast by waves and currents. The beaches and sea floor off Namibia and South Africa's Namaqualand became some of the richest gem diamond deposits known. Because only sound crystals survive the long trip, a very high share of these stones are gem quality, as covered on the Namibia diamond mines page.
Amber in sedimentary layers
Amber is fossilized tree resin. Resin dripped from trees, collected in soil or was washed into rivers, lagoons and deltas, and was buried in sediment. Over millions of years of burial, the resin hardened as its volatile compounds escaped and its molecules linked together. Baltic amber comes from sediment layers of Eocene age, about 40 million years old or more, along the Baltic coast. Dominican amber occurs in sandstones and shales in the mountains of the Dominican Republic. Burmese amber from the Hukawng Valley is much older, around 99 million years, from the Cretaceous period.
Turquoise, malachite and the oxidation zone
When rainwater and oxygen seep down into a copper deposit, they react with the copper sulfide minerals near the surface. This upper region is called the oxidation or weathering zone, and it hosts minerals that form only at low temperature in the presence of water and air. Geologists call minerals formed this way supergene minerals.
Turquoise forms here when copper-bearing water reacts with aluminum and phosphorus in the surrounding rock. It fills veins and forms nodules in altered rock in arid regions, such as Iran's Nishapur district and the American Southwest. Malachite and azurite, both copper carbonates, form in the oxidized zones of copper deposits, as in the Democratic Republic of Congo, Zambia and Russia's Ural Mountains. Chrysocolla, a copper silicate, forms in similar settings.
Chalcedony, agate and other low-temperature silica
Low-temperature silica also fills cavities in sedimentary rocks. Chert and flint form nodules in limestone and chalk. Jasper and petrified wood form where silica replaces or fills sediment and buried wood. The Petrified Forest in Arizona preserves logs turned to colorful chalcedony and jasper in sedimentary layers. Many agates form in volcanic cavities, but agate and chalcedony nodules can grow in sedimentary rocks too.
Frequently asked questions
What is a placer deposit?
A placer deposit is a concentration of heavy, durable minerals such as gold, diamond or sapphire in river, beach or sea-floor sediment. Water sorts the heavy minerals out from lighter sand and gravel.
Is opal a sedimentary gem?
Most Australian opal formed in sedimentary rock from silica-rich groundwater. Some opal, including Ethiopian and Mexican material, formed in volcanic rocks instead.
What is illam?
Illam is the Sinhala term for the gem-bearing gravel layer mined in Sri Lanka. It lies beneath clay and silt and contains sapphire, spinel, chrysoberyl and many other gems.
Why are alluvial gems often cleaner?
River transport breaks crystals along fractures and inclusions. The stones that survive the trip tend to be the soundest ones.