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| The most elaborate man-made shape is a cut gemstone. On the left is a Topaz, and on the right, a Cubic Zirconia made from Zircon. This is an imitation diamond. Gemstones are cut to maximise the light they reflect and refract, to make them shine and sparkle. They usually have many faces, and are symmetrical and regular. |
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A simpler way of cutting minerals is just to cut and polish a surface, or possibly a slice. This shows up the pattern of the stone beautifully. Agate is the most common example in rock shops.
Stones may be carved into different shapes as well. These may be animals or boxes, or a simple shape like an egg or a sphere, such as this Columbite. |
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| An even simpler method is tumble-polishing. Here, stones are rolled around in a machine until they wear each other smooth and polished. The shapes are often irregular, but the surface is very smooth and shiny. These sorts of stones are often the cheapest in rock shops, but they can be very attractive. On the left is Cornelian, which is common, and easy to identify because it's orange and translucent. On the right is Amazonite - bright green. |
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| All the above have man-made shapes. However, it is fun to see what the mineral looks like in its natural state. Sometimes it just looks like a lump of stone, although it may have an interesting colour or pattern. Turquoise and Snowflake Obsidian are on the left. The specimen on the right is Sulphur, which also has an interesting smell! |
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However, sometimes the mineral forms strange shapes. Left is red Hematite in its botryoidal form. This means it
comes in blobs. Another botryoidal mineral is green Malachite, where the layering of the blobs give its distinctive stripes of different shades of green.
There are even stranger shapes, as in this "desert rose" of Gypsum. |
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| Crystals are the most famous of natural mineral shapes. The word "crystal" comes from the Greek for ice, because the Ancient Greeks thought that Clear Quartz was ice that had hardened. Water can form crystals, as snowflakes! Quartz is what most people think of when they say crystal. The crystals have six parallel sides and then, at one end, slope towards a point or a short edge right at the top. All faces are very smooth and shiny. Indeed, it's hard to believe that this is natural. Sometimes quartz crystals are packed together so you don't see the parallel sides, just the point at the top, like this Amethyst. |
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| It is easy to think that crystals must be transparent or translucent, and must be long and thin, coming to a point. There are other crystals apart from quartz that can look like this, such as Calcite on the left. But crystals can be metal! The specimen on the right is Stibnite. |
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| The shapes do not have to be pointed. This Molybdenite crystal (left) is a flat metal hexagon. It is set in some other white mineral, so you can't see the whole hexagon. The Ruby (right) is flat as well. The emerald is stubby. |
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| The crystals don't have to be hexagonal (six-sided). These fluorite crystals on the left are cubes, although, since there are several of them joined together, and they are tilted to point upwards, they do not look like it. But the green fluorite on the right has more obvious cubes. |
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| Pyrite can have crystals of different shapes. On the left is nearly a perfect cube (although you can see a another tiny cube sticking out of it) and a dodecahedoncrystal. It may seem odd to call these crystals, but that is exactly what they are. On the right, the pyrite crystals are clustered together so you cannot see them very well. |
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Selenite has diamond shaped crystals. |
Spinel is an octahedron (eight-sided). |
So is Zircon. |
A cluster of Aragonite crystals. |
Manganite crystals are black, metallic and needle-shaped. |
Kyanite crystals are thin and flat, with no point. |
Tourmaline can be long, thin, but not pointed. |
A crystal of Copper. |
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| When two are more crystals grow at the same time, they can "twin". On the far left, there is a single Staurolite crystal. Next to it, two crystals has grown to form a cross. Andalusite (on the right) is another mineral which can make a cross. |
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| There is one more group of shapes, which while they are man-made, the shape is determined by the minerals. If you break a stone, different things happen depending on the stone. It may break very easily along a particular plane. This is called cleavage. Slate (left) can be split very easily into thin slabs, which is why it is used for roofing. Mica (right) has thin sheets, sometimes jumbled together. Clear mica has been used for windows. |
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Calcite has two cleavage planes. If you break a piece, you tend to get smooth surfaces parallel to each other without being a cube. You can see this in several of the specimens on the right. These are not crystals, it is just the way that the stone splits. |
Other minerals don't split so easily. The surface caused by the break is not smooth and flat. This is a fracture. There are different types of fracture. The surface may be rough, or it may be rippled and curved, like obsidian. Obsidian's fracture is called conchoidal, which means "shell-like". While cleavages and fractures are caused by someone hitting the stone, a particular mineral will cleave or fracture in a particular way, making one more way to identify it. |