The quartz on top of the agate are generally either white or transparent, but quartz may have coloration as well.
Geodes may be found all over, but they tend to be most prevalent in desert regions. Anywhere volcanic ash is a part of the landscape and places containing large amounts of limestone are ideal places to hunt for geodes. The State of Iowa has a significant geode deposit, even naming a state park after the stone. A large cave in Ohio called the Crystal Cave offers tours for visitors wishing to learn about geodes. Aside from desert and central regions in the United States, Mexico, Australia, and Brazil are common places where geodes are prevalent.
Crystal caves and vugs are the locations where geodes tend to form. A vug is a cavity that occurs inside rock with crystals lining the inside of the cavity. Vugs may form as a result of faulting or the collapse of rock, and they often contain secondary minerals inside of them. Crystal caves are often hidden and blocked by many tons of rocks, making them difficult to find. Scientists theorize that crystal caves form when salty sea water evaporated millions of years ago, leaving behind salty layers of sediment that eventually hardened into a crystal.
Scientists and those who enjoy geology consider geodes to be hidden treasures, waiting to be uncovered. Virtually anyone can choose a nondescript rock to crack open and find out what is hiding inside. Every geode is unique, and they vary widely in color, and crystal formation. Geodes can also be a variety of sizes from small to quite large. Opening a geode involves tapping it with a hammer. Some people prefer using a rock saw to cut the rock in half. It only takes a minute to sign up.
Connect and share knowledge within a single location that is structured and easy to search. Can you assist me in understanding how this hollow was formed in this rock.
I am told that it has to do with cementation but I believe that geological process produces rounded canonball sometimes like stones. Is this some sort of hollow or reverse cementation? I have also been informed that it was likely formed beneath the surface when the sandstone was forming into rock and it is a completely distinct process from that which the rock has since undergone when it has been exposed to weathering on the surface.
It looks to me that there were voids in this rock when it was formed, caused by gas bubbles These voids might have become filled with sand or other friable material. Weathering eroded the side of the hollow, enlarging it and allowing any finely divided material inside it to trickle out.
You should have included something to show scale, but my impression is that your rocks are about six feet tall and not very solid, therefore easily eroded. The round stones you refer to are geodes, which are hollow and usually encrusted in the inside with quartz crystals. In some parts of England they are referred to as 'potato stones', which gives you an idea of what they look like.
Some recent posts on this site give you photos of them. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more.
Geodes sell rapidly at gem and mineral shows, science museums, rock shops , art galleries and shops that feature international or natural gifts. When geode-opening demonstrations are given at rock and mineral shows, they always draw a crowd -- and that crowd usually produces enthusiastic cheers and gasps when a nice geode is opened.
There is something special about a beautiful treasure hiding in a rock that looks absolutely ordinary from the outside. Geodes in basalt: An outcrop of a basalt flow exposing several open amethyst geodes.
This photo illustrates how geodes can occur in large numbers within a single rock unit. Geodes are not found randomly here and there. Instead they are usually found in large numbers in areas where the rocks have formed in a special geochemical environment. Most geodes localities are in A stratified volcanic deposits such as basalts and tuffs ; or B stratified sedimentary carbonate deposits such as limestones and dolomites. A diversity of other environments yield a small number of geodes.
Geodes form in many different ways, and there are a number of valid theories about their formation. The intent of this article is not to provide a single or a comprehensive coverage of the many ways that geodes might form. The Story in the Rock: A spectacular sawn and polished geode specimen with multiple layers of colorful agate and a crystal-filled central cavity. Each colored band represents an episode of agate formation and a change in the composition of the groundwater that delivered mineral material into the geode.
The most widely known and sought-after geodes are those that formed in areas of volcanic activity. Voids in basaltic lava flows often are infilled with agate, quartz, opal and other material delivered by hydrothermal water or groundwater. Some voids are spaces occupied by gases that failed to escape the lava flow before its surface crusted over. Where does all of the gas come from?
Some magmas contain a lot of dissolved gas. They can be several percent dissolved gas on the basis of weight. Think about that - several percent gas by weight!
When these magmas ascend to the surface, the gas expands in proportion to the pressure reduction. When the magma erupts as a lava flow, so much gas is released that not all of it is able to escape. Some of that gas can be trapped in the lava to produce a large cavity when the lava solidifies.
Other voids in solidified lava flows were produced as liquid lava flowed out after the flow was only partially solidified. These small "lava tubes" produce some of the largest and longest geodes. Many cathedral geodes are branches of these lava tubes that later infilled with mineral material. Many of them have the geometry of long tree branches, being nearly a meter in diameter and many meters in length.
Breaking a Geode: An exhibitor at a gem and mineral show preparing to break a geode. The device used for breaking geodes looks like a long-handled bolt cutter. However, the pair of blades used to shear the bolt is replaced by a chain that is tightened around the geode until it breaks cleanly in half. Geode-cutting demonstrations usually attract a cheering crowd at gem and mineral shows. Image by ilovebutter, used here under a Creative Commons License.
Geodes in sedimentary rocks are usually found in limestones, dolomites, and calcareous shale. In these deposits a gas-filled void can serve as the opening for geode formation. Shells, tree branches, roots and other organic materials often decay away to leave a void for the formation of mineral materials.
These cavities can be filled with quartz, opal, agate or carbonate minerals. They are generally smaller than the geodes formed in volcanic rocks. Geodes are most easily collected when their host rocks have weathered away. This can occur because basalt, limestones, dolomites, and shales weather much more readily and rapidly than the quartz and chalcedony that typically form the outer layer of a geode. The host rock weathers away and the geodes are left on the surface, washed into a stream, or stranded in a residual soil.
In these situations the geodes are easily found and collected. Some geodes are produced by mining the host rock, but that method is difficult, costly, and often damages the geode. Geodes are given a variety of names. The word "geode" is often preceded by the name of the mineral material which has filled the geode. The word "geode" might also be preceded by a geographic or stratigraphic name. Geodes ready for shipment: Geodes from Brazil and other collecting localities are carefully crated to prevent damage during shipment.
This photo shows a matched pair of cathedral geodes which are two halves of the same cavity. They have been given bases of heavy concrete that will enable them to be used as items of decor in a home or office. An unopened geode has the appearance of an uninteresting rock. They become much more interesting when they are opened and their internal crystals and agate bands become visible.
And even though every crystal-lined geode is a wonder of nature, there are many things that can be done to make it into a more marketable product as well as enhance its value.
But, before we go any further, we want anyone who finds a really nice geode to know that museum-quality geodes are sold in the collector market for prices that will astound the average person. The Arkenstone, a business that specializes in museum-quality mineral specimens, usually has a few nice geodes for sale. At the time this article was being written, they were selling rare-mineral lined geodes featuring volkovskite, millerite, thomsonite, and shattuckite for four- and five-digit prices.
You can see some of their inventory here. Amethyst geode sections: Small sections of amethyst geodes offered for sale in a tourist shop. Large agate geodes are often cut into blocky pieces with a flat base to make beautiful bookends, desk sets, clock faces or paperweights. Those with gray, white, or other less-interesting agate colors are often dyed bright blue, green, purple, red or other colors and then face polished to make them more appealing.
These treatments make the bookends sell faster and enhance their value beyond what would be paid for the less-interesting gray color. Large amethyst-lined geodes are often expertly cut to display their internal amethyst crystals. They are then sawn and fitted with a weighted base to enable them to be used as an item of home or office decor.
Agate-lined sections of lava tube geodes have frequently been used to make "cathedral geodes" that are several feet high. These are cut in a way that nicely exposes the amethyst-lined chamber, they are then sawn to produce a flat base, which is filled with metal-weighted concrete to enable the geode to stand upright, the edges of the cavity are sanded for nice appearance, and the entire exterior is painted to hide the scrapes and scuffs that occurred during collection and transportation.
Many of these large geodes sell for thousands of dollars each. Dyed Geode Slices: These dyed geode slices have been mounted in wooden bases for display. Small geodes are often sliced and polished. Especially nice slices might be displayed "as is" in specially-made frames or stands. Some have their translucent beauty displayed in stained-glass panels or windows like the example photo shown at the top of this page.
Less spectacular specimens might be dyed and used to make wind chimes, coasters, or decorative magnets. Small sections of geodes with attractive and colorful crystals are often sawn into small pieces that will stand upright or into slabs that are used as display items.
Small Geodes: Small geodes like these are often sold in novelty and science stores to people who want to have the experience of opening a geode themselves. They want to be the first human to see the treasure inside. Small, thin-walled geodes are often sold in kits by department, education, science, tourist and novelty stores.
The vendor product descriptions encourage teachers, parents and students to purchase the kits and break the geodes open with a hammer. These geode kits are extremely popular. If you do a Google search for " geode kit " you will find them offered online by dozens of different vendors. If you are tempted to purchase one of these kits, be sure to read the reviews because quality varies greatly. It is also essential to plan for proper safety equipment, because striking a geode with a hammer can produce flying rock fragments that can cause serious injury.
You will also need a good place to do this activity. Following some of the vendor suggestions of breaking them on a table, or classroom floor, can cause property damage that is costly to repair. An alternative to purchasing a "break-open-a-geode kit" every year is to purchase a few specimens of opened geodes and sliced geodes.
0コメント