What is a rock, Exactly ? After reflection and discussion, most people will agree that rocks are more or less hard solids, of natural origin and made up of minerals. But for geologists, all of these criteria have exceptions.

Are all rocks hard ?

Not necessarily. Some common rocks can be scratched with your fingernails, such as shale, soapstone, gypsum and peat. Others may be soft in the ground, but they harden once they spend time in the air (and vice versa). And there is an imperceptible gradation between consolidated rocks and unconsolidated sediments. Indeed, geologists name and map many formations that are not made of rock at all. This is why geologists refer to working with igneous and metamorphic rocks as “hard rock geology.”, as opposed to “sedimentary petrology”.

Are all rocks solid ?

Some rocks are far from completely solid. Many rocks contain water in their pores. Many géodes – hollow objects found in limestone regions – retain water inside like coconuts. Two barely solid rocks include fine threads of lava known as Pele's hair and the fine open mesh of reticulite burst lava .

Ensuite, there is the question of temperature. Mercury is a liquid metal at room temperature (and until -40 F), and oil is a fluid unless asphalt bursts into cold ocean water. And good old ice cream meets all the rocky hood criteria too… in permafrost and glaciers.

Are all rocks natural ?

Not entirely. The longer humans stay on this planet, the more concrete accumulates. Concrete is a mixture of sand and stones (aggregates) and mineral glue (cement) calcium silicate compounds. It is a synthetic conglomerate, and it acts like natural rock, found in river beds and on beaches. Part of it entered the rock cycle to be discovered by future geologists.

The brick is also a artificial rock – in this case, an artificial form of massive slate.

Another human product that closely resembles rock is slag, the byproduct of metal smelting. Slag is a complex mixture of oxides that has many uses, including road construction and concrete aggregates. He has already found his place in sedimentary rocks .

Are all rocks made of minerals? ?

Many are not. Minerals are inorganic compounds with chemical formulas and mineral names such as quartz or pyrite. Coal is made of organic matter, not minerals.. The different types of substances contained in coal are rather called maceral. Likewise, what about the naughty… a rock made entirely of shells ? The shells are made of mineral material, but they are no more minerals than teeth.

Finally, we have the exception of’ obsidian . Obsidian is a rock glass, in which little or none of its material has gathered into crystals. It is an undifferentiated mass of geological materials, a bit like slag but not as colorful. Although obsidian does not contain minerals of its own, it is undoubtedly a rock.