The word ‘marble’ is derived from the Greek word ‘marmaros’ ("μαρμαρος"), which means ‘shining stone’,
The geological definition of marble is a metamorphic rock resulting from the metamorphism of limestone, composed mostly of calcite. More general the natural stones that are polishable are also referred to as marble.
Imitating marbles by painting (marbling) came into existence for several reasons.
For one because certain marbles were already exhausted, plus an imitation is cheaper than the real stone. And it has the additional advantage that it’s applicable almost everywhere, whereas using the real heavy marble sometimes can be impossible construction wise.
We can divide marble roughly into three categories, based on visual aspects:

1: Veined marbles.
The underground is usually monochrome with veins running through like cracks in a contrasting colour. These cracks are caused by the high pressure that’s on the stone, after which these gaps fill themselves with organic sediments and minerals that donate the colour.
Vert de Mer and White Carrara are familiar examples.

2: Brèche marbles:
Breccia is a rock composed of angular fragments of several minerals or rocks in a matrix, that is a cementing material, that may be similar or different in composition to the fragments.
Examples are Grand Antique and of course Brèche Violette.

3: Cloudy marbles:
These marbles are recognizable by their fragments in the shapes of clouds or rings twisted together. These shapes (often fossils in the underground, called ‘tortillons’) are accomplished by multiple small veins and some break lines.