What makes a stone


Whether green emerald, blue sapphire or red ruby: we know precious stones as sparkling and particularly valuable pieces of jewelry. Gemstones are simply minerals. However, they have to meet three requirements in order to be considered gemstones: They have to be particularly rare, transparent and at the same time very hard.

Gemstones are formed deep in the earth's interior under high pressure and at high temperatures. The hardest of them and at the same time the hardest known mineral is diamond. It is formed from a single element at a depth of around 150 kilometers at temperatures above 1200 degrees Celsius: carbon. Crystals usually develop from eight equilateral triangles, called octahedra. Other shapes such as cubes are also possible. The diamond gets to the surface of the earth by being thrown upwards together with the rising magma. The largest diamond ever found is the so-called "Cullinan". It was discovered in a South African mine in 1905 and weighed exactly 3106.75 carats in its raw state. This corresponds to a weight of 621.35 grams.

Whether diamond, amethyst, emerald or topaz - all gemstones differ from one another in terms of structure, composition and color. They only become particularly beautiful and shiny when they are cut. He lets the colors of the gemstones shine through a certain refraction of light.

In addition to precious stones, there are other gemstones in the earth's crust, such as blue lapis lazuli or green malachite. Although these are also very sought-after and beautiful, they are not see-through and are too common to be considered gemstones.


In a mine in South Africa, a rough diamond was found on Thursday that is larger than all known diamonds on earth. This record gem weighs a total of 3106 carats. He was discovered by Frederick Wells, the mine inspector.

It happened in the late afternoon during a routine inspection of the Premier Mine near Pretoria. On his tour of the mine, Frederick Wells saw something light up above him. Wells became curious and took a closer look. Could that dazzling thing really be a giant diamond? He managed to pull the stone out of the wall and still wasn't sure if it wasn't a piece of glass that was making a fool of him. However, research has proven that he was really dealing with a diamond. And with the largest one that has ever been discovered on earth. The valuable chunk has exactly 3106.75 carats. That corresponds to a weight of 621.35 grams or a little more than 6 bars of chocolate.

For his sensational discovery Frederick Wells is now to receive a "finder's reward" of $ 10,000. The name giver for the greatest diamond of all time is mine owner Sir Thomas Cullinan: The famous "Cullinan diamond" is already being mentioned everywhere.

The "Immortal"

Its sparkle drove many people crazy - or at least a fortune. No wonder, because the diamond is the most valuable of all gemstones. And not only that: it is the hardest known mineral. That is why the dazzling top carat is also called “the immortal”. Myths and legends surround this rare jewel. Famous diamonds such as the "Hope" or the "Florentine" are said to have a curse that brings bad luck and death to their owners. Others, however, see the diamond as a symbol of eternal love, wealth and power.

Metamorphic rocks

It happens inside the earth: Strong pressure and high temperatures ensure that the constituents of the rock, the minerals, react with one another and transform. In this way new rock is formed. Because the Greek word for metamorphosis is “metamorphosis”, geologists also speak of metamorphic rocks.

A correspondingly high pressure is created when two earth plates collide and one plate dips under the other. The rock is then squeezed together like in a huge press. A frequent result of such a rock metamorphosis is the blue schist. Its parent rock is basalt or a rock with a similar composition to basalt.

Extreme heat also causes rocks to transform. For example, it is baked near a magma stove like in an oven. Marble, for example, is nothing more than limestone that has been heated very strongly in the interior of the earth; During this process, new minerals are formed and the rock becomes harder. Sandstone also changes at high temperatures, because its quartz grains then stick together: the harder quartzite is made from the original sedimentary rock.

In contrast to the complete melting through volcanism, the rock remains solid during the metamorphosis. However, if the temperature continues to rise, the rock will eventually turn into liquid magma. If this mass cools down, it turns into igneous rock. The rock cycle is in full swing.

Cycle of rocks

No rock on earth is made to last. It weathers on the surface, is removed and redeposited. When two plates collide, layers of sediment are compressed and unfolded to form high mountains. The rock of submerged plates melts in the earth's interior and forms the source of volcanoes. Lava that spits out from a volcanic crater cools down and solidifies again into rock.

It is an eternal cycle that ensures that even the hardest rock is constantly changing and new things are created from it. The transformation does not happen overnight, of course, but over millions of years. "Players" in this cycle are three groups of rocks, each of which is formed under different conditions:

When magma cools, the hot mass solidifies igneous rock. This can happen both on the surface of the earth and inside the earth. On the other hand, where layers of excavated rock pile up, the sediments are compressed under the weight of their own weight. This pressure causes them to solidify Sedimentary rock. In turn, high pressure and great heat in the earth's interior ensure that rock is transformed and another is created. Then geologists speak of transformation or of metamorphic rock.

These three types of rock are closely related: each type can transform into any other. This rock cycle will continue as long as the earth exists.

From inside the earth: ores and solid metals

Copper was the first metal that humans discovered in the earth's crust. It could be shaped into simple tools or weapons and was so important that an entire epoch was named after it: the Copper Age. The tools got better when man mixed the copper with tin and thus invented the bronze. And when he learned to smelt iron, the triumph of metal tools finally began.

In contrast to the earth's core, the earth's crust consists largely of non-metals. Nevertheless, metals such as iron, aluminum, manganese and potassium can be found in their rock. Experts (geochemists) can determine exactly how often they occur. They found out that around seven percent of the earth's crust consists of iron.

Like most metals, iron occurs as a chemical compound with other elements, so-called ore. In order to extract iron from the ore rock, the ore rock is ground, mixed with coal and heated. Then a chemical reaction takes place that removes the other elements from the ore, leaving the pure, elemental iron.

On the other hand, some metals hardly combine with other elements. They therefore do not weather and occur in pure form in the earth's crust. These “solid metals” include gold, silver and platinum. Platinum and gold are also extremely rare: gold is only contained in an average of 0.001 grams per ton of rock. A place is only referred to as a deposit if it contains a thousand times the amount of gold - i.e. one gram of gold per ton of rock.

The "rare earth metals" are more common than gold or platinum. What sounds strange has a simple reason: These metals are considered rare because they do not form their own deposits, i.e. they do not occur in concentrated form, but only in scattered areas. We are therefore also talking about spice metals. Their importance has increased significantly in recent years because they are required for the manufacture of electronic devices such as cell phones or computers.