According to scientific terminology, this indicates a type of rock known as chalcedony consisting of alternating layers of cryptocrystalline silica.
In commercial terminology, on the other hand, it indicates a natural compact and polishable stone, with a calcitic composition, consisting of alternating layers of precipitation of calcium carbonate from saturated waters, which may also be "enriched" by chromophorous elements thanks to which the rock acquires peculiar colouring, bands and "cloud" patterns. 
The most extraordinary feature of rocks of this type is their exceptional transparency which makes the rock translucid; that is why it was already being used for refined glazing at the time of the pharaohs.