xeyla

Originally Posted by Adversary

Gold is usually found with quartz (but unfortunately not the other way around) and granite which is are igneous materials so there would have to have been some magma near the surface of the earth. I have also heard that some gold was found in meteor impacts but I can't believe that would be a major source.

Marble on the other hand is metamorphosed sedimentary rock high in carbonates (think limestone). The original limestone was formed on a seabed with lots of shell forming creatures. The limestone must then be buried deep enough for the pressure and heat to bake (or pressure cook) the limestone into marble. You may find both types or rock in the same area, but unless there was a lot of deformation of the landform, you will not find marble under granite. Granite (or more likely Gneiss) would lay below the marble.

Above the marble, or by itself, you may find limestone. Limestone makes a great building material and is valuable in its own right. It is useful for gravel, walls, sculptures or for making cement.

Iron veins formed from the oceans. Iron is highly soluble in water and likes to be there. But iron likes oxygen even more and will leave water in a heartbeat if there is enough oxygen around. Before the earth had a high concentration of O2 most of the iron was in the oceans. Once the O2 level reached a critical point it joined with the iron to form iron-oxide which precipitated out of the sea water to form layers on the seabed. When the sea level dropped, or the land was uplifted it left these bands of iron available for relatively easy mining. These bands are again often found with limestone.

Many useful materials or minerals are found in mountains. Not because they are only found in mountains, but because it is easier to find them exposed. It would be possible to find gold, silver, copper, iron, marble and limestone in one mountain, but not likely.

Tin is another metal found in Granite deposits. As I said before Granite is an intrusive rock formed by magma cooling underground. It is rich in many metals, rich being a relative term. Tin is normally found, like iron, in its oxidized state in the ore Cassiterite (SnO2). It was most likely found on the surface or in streams as it weathered out of granite exposures (like gold). It is also apparently often found with copper. I read that the earliest bronze items only has trace (2-3%) of tin in the mix. It is possible that this was how the alloy was discovered. For our purposes (in a fantasy or ancient world) Tin, Copper, lead and gold would be found on the surface downhill from granite mountains. The mining would most likely be surface mining or mountain side mining where a vein of the desired ore was initially exposed by erosion. Deep mines or strip mines would be rare but possible. It is interesting to me that tin is much more rare than iron in the earth's crust. Tin makes up 2 parts per million of earth's crustal material were iron makes up 50,000 ppm.

Copper mining would be similar to the other metal mining. The surface deposits would be used first, then shallow mining and mountain side mining would follow.

Another easily mined substance that is often overlooked is kaolin. Kaolin is a high quality clay. It was deposited in slow moving or still water. Clay particles are so fine that it requires very little water movement to move them. They can collect in tidal flats, swamps or river deltas. Clay can be found near or in still water or in some places on the surface.

Gems, Jewels and other shiny stones. There are too many to list all of them so I will select a sample of useful or popular stones. I will start with the obvious jewel the diamond. Most people know that diamond are pure carbon, not so many people know that diamond can burn and dissolve. Forming carbon into diamonds requires heat and pressure, but too much heat and the carbon will burn. For these reasons diamond are mostly found in mines, but of course do appear on the surface in the pipe where they formed... usually. There was a diamond rush in Wisconsin until it was realized that the diamond pipe was actually in Canada and the diamonds were relocated by the glaciers during the ice age. Speaking of glaciers, ice is a mineral and can be quite valuable especially in warmer climates. Ice can be mined from glaciers or frozen lakes. The ice blocks are then packed in sawdust and put in ice houses for the summer. Ice from natural sources was a valuable commodity until well the 1920's.

Amber is another exception to the valuable stone rule. Amber is not really a stone but is petrified tree resin. Amber is a lot more common than you might think. Amber is often found with coal but the grains are usually sand sized and not worth anything (more on coal in another post). Amber is not always.. well... amber in color. It can come in shades of blue, red or black as well. While mostly used for jewelry, amber has also been used as a medicine and in perfume.

Quartz, is the second most common mineral on earth. It is found with... you guessed it, granite. Chemically quartz is SiO4 which forms a clear crystal, but impurities can give quartz different colors and properties. Varieties of quartz include: rose quartz, tiger eye, amethyst, citrine, agate and many more. Quartz crystals of unusual size and clarity are thought by some to have magical properties and my be used in some types of healing.

Jade is actually two different rocks. Nephrite and jadite, they are both metamorphic silica minerals. Jade comes in a wide variety of colors, but green is the most well known. It is usually weathered out of... not granite. Jade is a metamorphic rock and granite is not. But, granite or sedimentary rocks can be metamorphosed into gneiss (nice) and that is where jade is found (as well as garnets). Jade was/is used for its mystical properties as well as in art and jewelry.

Rubies are cool because we don't know exactly how they form in nature. Rubies are a form of corundum and are made from aluminum oxygen a touch of chromium (for color) They form where there is little silica (the most abundant mineral) and iron (the most abundant metal). They are most often found in marble deposits. Rubies are slightly fluorescent. Under UV light (like in sunlight) they give off more visible light than they absorb. Burmese warriors would imbed rubies under their skin so they would be immune to injuries in battle. Rubies were also used in medicine for flatulence, stomach pains and indigestion.

Sapphires are similar to rubies, take out the chromium and add a bit of titanium, magnesium, copper or iron for color. They come in colors from red to yellow and blue. They are found in alluvial deposits. Sapphires do not erode easily so they fall out of the rock matrix where they formed and drift down stream until they are deposited in a loose matrix (alluvial deposit). They may be found with gold, platinum or other gem stones (a valuable alluvial deposit is also known as a placer deposit).

This is useful info.