Igneous rocks are called fire rocks and are formed either underground or above ground. Underground, they are formed when the melted rock, called magma, deep within the earth becomes trapped in small pockets. As these pockets of magma cool slowly underground, the magma becomes igneous rocks.

Igneous rocks are also formed when volcanoes erupt, causing the magma to rise above the earth's surface. When magma appears above the earth, it is called lava. Igneous rocks are formed as the lava cools above ground.

Sedimentary

As rocks began to erode from wind, rain, and sun, the particles were washed into low lying areas where the sediment accumulated. In the sea, small plant and animal forms died and drifted to the bottom, adding to the sediment. Over thousands of years, the pressure of these layers cemented the sediment onto limestone or sandstone.
Limestone formed on the sea floor from sediment and the bodies of primitive sea creatures. As you are working, you can find small fossilized creatures like crynoids and braciopods in the stone. Limestone is composed primarily of the mineral calcite, or calcium carbonate.
Limestone is easy to carve and will hold small detail work, but is also strong enough to support undercutting. While it does not have the innate beauty of the more colorful stones, a wide variety of textures can be created that can really bring the stone to life. There is a definite 'grain' or bed (formed as the layers of sediment stacked upon each other on the sea floor) in limestone. The stone breaks fairly predictably along the bed lines, but less cooperatively going across the bed. (Think of opening a phone book with the pages, compared to trying to tear it across the pages.) Limestone can be polished, but the polish will not last long outdoors. Limestone seems to survive acid rain better than marble, so is a good choice for outdoor sculpture.
Sandstone is formed from sedimentary sand held together by silica or calcium carbonate. Sandstone also has a distinct bed direction, and large chips can be broken off when going with the bed. Sandstone tends to wear out your tools quickly. Sandstone contains silica, so a respirator must be worn.

For thousands, even millions of years, little pieces of our earth have been eroded--broken down and worn away
by wind and water. These little bits of our earth are washed downstream where they settle to the bottom of the rivers, lakes, and oceans. Layer after layer of eroded earth is deposited on top of each. These layers are pressed down more and more through time, until the bottom layers slowly turn into rock.

Metamorphic

Metamorphic rocks are formed when a sedimentary layer is exposed to heat and pressure and undergoes a chemical change which forms a new crystalline material. After metamorphism, limestone becomes marble.
Marble has been the most preferred stone for carving since the time of the ancient Greeks. Marble is moderately hard to work. It will hold very fine detail. Marbles from the United States comes in over 250 colors. When brought to a high polish, its crystalline structure sparkles. It does not hold up well outdoors. Acid rain begins to deteriorate the surface within a few years.

Metamorphic rocks are rocks that have "morphed" into another kind of rock. These rocks were once igneous or sedimentary rocks. How do sedimentary and igneous rocks change? The rocks are under tons and tons of pressure, which fosters heat build up, and this causes them to change. If you exam metamorphic rock samples closely, you'll discover how flattened some of the grains in the rock are.