what makes a mineral a gem?

Question

please help!

Answer 1

Heat and pressure.

Answer 2

It's basiclly just the beauty. Desirable traits are durability, rarity, and ability to be worked into jewelry, but there are exceptions to every one of those features. Some gems (pearls, amber, etc.) are not even considered minerals by lapidarists and gemologist.

Answer 3

it's aesthetic properties. that's it - purely if it's pretty, or not

Answer 4

Following is a tutorial from the mentioned web site, the web site also has some complimentary graphics:

Mineral Crystallization
... The earth's crust varies from 3 miles, (5k,) thick under the seabed to 25 miles, (40 kilometers,) under continents. Under the crust is the mantle, which is approximately 1860 miles, or 3000 kilometers thick. It makes up 83% of the earth's volume.
... The mantle is composed of molten rock, called magma. In the rare occasions where it reaches the surface, we call it lava. The mantle is hottest near the center and heat currents keep it in constant motion.
... Where the mantle and crust meet, is a tumultuous zone with high pressures and temperatures. The crust is made up of several plates that float on the liquid mantle. As they run into with each other, some are pushed down; others are raised into new mountains.
... The magma is also in constant motion. Its movement and pressure are constantly acting on the bottom of the crust, creating wear and fracturing. Rocks break free from the crust and are carried away in the fluid magma. Much of this material melts, changing the chemistry of the nearby magma. Some of the smaller particles are destined to be inclusions in future gems.
... The lower surface of the crust is heavily fractured and contains numerous cavities. Fluids escaping from the magma flow through these fractures and cavities.
... It is here we find the proper conditions for crystal growth. The fluid is a chemical rich soup, which supplies the necessary ingredients. The cavities offer space to grow and the temperature and pressure are high. As the fluid moves through the crust, it cools enough for crystallization to occur. Only time is still required.
... One would think that, in geological terms, time should be more than sufficient. However, this is a highly tumultuous environment. The passages are constantly opening and collapsing. Often crystals start to form, and then the passage feeding the fluid is closed off. At this point all growth stops.
... If the passage reopens, growth will begin again. Often this on and off growth pattern is undetectable in a crystal. Other times, the successive layers of growth will have a slightly different chemical composition. When this happens, you see color zoning in the crystal.
... In some occasions, the new layers will have different orientation. This is the cause of twinning. Still other times, the new layers will not bond completely with each other. When you see parting on a star ruby, it is because the layers did not bond.
... There is nothing that says that, in the on and off growth process, the same minerals need to reform. Indeed, the temperature, pressure, and/or chemistry often vary, producing different minerals. When opening a deposit, it is common to see different minerals covering earlier layers.
... This is also one cause of inclusions. A new crystal may start to grow on an older and larger one, only to have the growth process stop. If the original crystal begins growing again, it will be over the newer ones.
... In a few unique situations, a nice quartz crystal form. Some time later, the chemistry changes and a very fine layer of, maybe feldspar, will cover the quartz. Still later, the conditions change again and the original quartz crystal grows more. The result is a phantom crystal.
... Sometimes two different minerals will crystallize at the same time. If one takes off and starts growing faster, it will engulf the other. This is how pyrite crystals end up inside emerald.
... In still other conditions, there will be chemical impurities within a crystal. If the temperature and/or pressure change, the impurities can crystallize inside the host crystal. This is how rutile forms in quartz and corundum.
... During the rough and dramatic changes in the crust, many crystals are broken. If the conditions for growth are present, material will seep into the fractures and crystallize, "healing" the fracture by growing back together. However, they never heal completely and fine cavities of gas remain in the previous gap. We see these as fingerprints and this is why they are also called "healing fractures."
... There are tremendous pressures in the crystal growth environment. Many crystals are compressed beyond their natural size.
You can see this in a polariscope. Many gems show strain, as rainbows of color. Sometimes this causes stress fractures.
... Strain also makes leaves stone subject to breakage. Many faceters have placed a stone on a lap, only to have it shatter. The forces inside the stone literally cause it to explode.
Getting Specific
... Mineral creation is now fairly well understood. Advances in geology and synthetic gem manufacturing have unraveled many of nature's mysteries.
... Traditionally, we were taught that there are three kinds of rock formation; igneous, metamorphic, and sedimentary. Igneous minerals are created with heat. They are minerals that are created deep within the earth. Metamorphic refers to conditions where heat and pressure change existing minerals into something new. Sedimentary rocks are based on deposits of sediment.
... Today, geologists prefer to describe rock formation as four processes: molten rock & associated fluids, environmental changes, surface water, and gems formed in the earth's mantle. Not that mineral creation is simple and straightforward. They are continuously being destroyed and recreated as in the "Rock Cycle" chart below.
Molten Rock & Associated Fluids
... Technically, gems rarely form in the magma itself, but rather from fluids that escape from it, (pegmatites and hydrothermal.) The two exceptions to this are called magma and gas crystallization.
Magma Crystallization
... Magma contains a variety of elements. As it cools, the elements combine to form minerals. Exactly what mineral is created varies with the available ingredients, temperature, and pressure. Each time one mineral forms, the available ingredients change. Different minerals form as it goes through the various stages of changing temperature, pressure, and chemistry.
... Unless the conditions are just right, crystals will not form. Instead, it will simply cool into a solid mass of small, interlocking crystals; what gemologists call an aggregate.
... In some occasions, one mineral will crystallize nicely. Then, before any more crystals can form, the magma will find a break in the crust and rust towards the surface. Here the pressure and temperature are too low to allow crystallization. Instead, the rest of the magma cools into fine-grained rocks, with the original crystals distributed through out the interior. These are called phenocrysts.
... Corundum, moonstone, garnet, and zircon are often found as phenocrysts. The Chantaburi and Trat districts in Thailand have large deposits of ruby and sapphire phenocrysts.
... Diamonds crystallize at temperatures higher than other minerals. Scientists now believe that they may form in the magma, near the earth's crust where it is the coolest. If this is true, it also means that conditions for diamond crystallization are the most common in the earth. Diamonds may actually be the most plentiful crystals in the earth, just not the easiest to reach.
Gas Crystallization
... Have you ever wondered why some crystals are doubly terminated, where most are broken off at the base? Most crystals grow on a solid base of other minerals. However, a few actually grow inside gas bubbles!
... These gems form after the magma has reached the surface. During a volcanic eruption, rising magma undergoes a rapid reduction in pressure. This causes gas bubbles form, just like removing the cork from a bottle of champagne.
... Sometimes these bubbles will contain high concentrations of certain elements. If the right combination of temperature and pressure exist for a long enough time, crystals form. Garnet, topaz, and spinel also form this way.
... One of the best known examples of gas crystallization are "Herkimer Diamonds," the water clear quartz crystals from Herkimer, New York.
Hydrothermal
... As the name implies, hydrothermal involves water and heat. As water percolates through the earth, it dissolves minerals, just as it did with the sugar in our rock candy. Deep inside the earth, it meets with magma. Special fluids then escape from the magma that contain water, carbon dioxide and volatiles, (substances that give off gasses.)
... These hydrothermal fluids move through fractures in the crust. Along the way, they may dissolve minerals, or combine with other ground water. Mineral rich, they begin to cool in "veins." If combined with the right combination of temperature, pressure, time, and space, crystals form.
... Hydrothermal deposits are special, because they can have combinations of elements not found elsewhere. One of the most important hydrothermal deposits is the Muzo emerald field in Colombia
Pegmatites
... Sometimes magma in the upper part of the mantle becomes concentrated with volatiles. This volatile rich magma is sometimes forced into a cavity where it cools. This is the definition of a pegmatite. It differs from a hydrothermal vein, in that magma is the primary agent, rather than water.
Environmental Changes
... Great stresses exist inside the earth. Under the right conditions, the temperature and pressure can rise to the point where existing minerals are no longer stable. Under these conditions, minerals can change into different species without melting. This is known as metamorphism.
... There are two types of metamorphism, contact and regional.
Contact Metamorphism
... Contact metamorphism occurs when magma forces its way into an existing rock formation. Under the intense heat, existing rocks begin to melt and eventually recrystallize as new species that are stable at higher temperatures.
... Sri Lanka is one of the best known sites of contact metamorphism. Garnets, corundum, and spinel are also common here. Lapis lazuli, which in found in the mountains of Afghanistan, is another stone created by contact metamorphism.
Regional Metamorphism
... Regional metamorphism takes place on a much broader scale and affects a much greater variety of minerals.
... The earth's surface is composed of large pieces, called "continental plates." Looking at them from a geological time frame, they are floating on the mantle and in motion. However, they do not all move in the same direction and some of them are actually competing for the same space. Where these huge structures are forced together, one is shoved under and the other is pushed up. This is our primary mountain building method.
... Enormous compression forces exist where these land masses come together, creating an area of intense heat and pressure. As the temperature approaches the melting point of rock, the minerals become unstable. Over time, (possibly millions of years,) they change into new varieties.
... East Africa is an excellent example of regional metamorphism. Minerals are found here that do not exist anywhere else. Tanzanite is a prime example, as are the unique varieties of garnet. Polymorph Minerals that share the same chemistry, but have different crystal habits. Pseudomorph
Minerals that have changed chemistry without changing crystal
form.
... During metamorphism, some minerals simply change habit. The same ingredients recrystallize in a new crystal system, as a new species. (Remember that a mineral is defined by a combination of its chemical make up and its crystal habit.) These are called polymorphs.
... For example, andalusite, kyanite, and sillimanite all have the same chemistry, Al2SiO5. They regularly polymorph by changing into other crystal systems.
... Other crystals will change chemistry during metamorphism. They may recrystallize in their customary habits and show no abnormal properties. However, sometimes a crystal will change chemistry without recrystallizing. These unique minerals are called pseudomorphs. A pseudomorph is an atom-by-atom replacement of one mineral for another, without changing the original mineral's outward shape.
... A prime example of a pseudomorph is tigers eye. Crocidolite has been replaced by quartz, but they retain the fibrous structure of crocidolite. Marcasite has pseudomorphed pyrite, gypsum, fluorite, and goethite. Malachite frequently pseudomorphs azurite, leaving a perfect azurite crystal shape that is composed of malachite.
Surface Water
... Rain plays an important roll in recycling minerals. Erosion breaks down rocks and moves them to new locations. Once on the ground, rainwater is instrumental in creating new gems.
... As water passes through the earth, it picks up chemicals that turn it into a weak acid. If heated, or mixed with the right chemicals, it can become highly corrosive. That gives water the ability to dissolve even more minerals.
... As water percolates through the earth, it picks up many ingredients. At times, it becomes too saturated to carry any more, so it leaves the excess in cracks and pores of existing rocks. This is how fossils and petrified wood are created.
... In other conditions, the water encounters combinations of minerals that create a chemical reaction. The dissolved minerals are then deposited as new minerals in seams and cavities. This is how opal, turquoise, azurite, and malachite are created.
... During the cretaceous period, much of central Australia was covered by an inland sea. When it dried, it left the area layered with silica rich sands. For millions of years, rain has been dissolving the silica. During the hot, dry summers, the ground water evaporates to the point where the remaining water cannot hold the silica in suspension. The excess is deposited in seams and cavities, not far below the surface. These silica deposits are opal.
... Turquoise, azurite, and malachite all receive their color from copper brought by water. The copper rich water must pass through limestone to create azurite or malachite. Turquoise requires that the water also picks up some phosphorous along the way.
Gems Formed in the Earth's Mantle
... Knowledge of the earth's mantle is still rather limited. However, the evidence shows that some gems actually form in the mantle. To do so, they need to crystallize at an extremely high temperature.
... The most notable examples of gems formed in the earth's mantle are diamond and peridot. By studying the peridot deposits in Arizona, geologists now believe they were created on rocks floating in the mantle, approximately 20 to 55 miles below the surface. An explosive eruption brought them near the surface of the earth. Weathering and erosion finally brought them close enough to the surface for people to find them.
... Diamonds are better understood. As mentioned before, diamonds actually crystallize in the magma below the crust. However, the magma formations they are found in have a different chemical composition. It is believed to come from greater depths, 110 to 150 miles below the surface. At this depth, the temperatures are higher and the magma is very fluid.
... This hot and fluid magma has the ability to force its way through the earth's crust faster and more violently than other volcanic eruptions. During the eruption process, it will break up and dissolve rocks from the lower mantle, and then carry them to the surface.
... If the magma rose slower, the diamonds would probably not survive. The changing temperatures and pressure would cause them to vaporize or recrystallize as graphite. It is believed that the speed of the magma rise is so quick they do not have the time to transform.
1 Magma pocket comes in contact with a weak area in crust.
2 A quick explosion results, carrying diamond-bearing magma to the surface. During the eruption, a cone builds on surface.
3 The pipe eventually cools, leaving carrot shaped pipe.
4 The cone quickly erodes away, (geologically speaking,) leaving the diamond bearing earth where people can reach them.
Coming Up
... Since crystals form so far under the surface, you may be wondering how they get to the top where people can mine them. A few crystals are brought to the surface during volcanic eruptions, as described above. However, most reach the surface through mountain building and erosion.
Over vast periods of time, the movement of the continental plates causes mountains to rise.
... Years of weathering take down the mountain, leaving the deposits near the surface.