Marble

Marbles are limestones or dolostones that have been metamorphosed. Since metamorphism recrystallizes the individual calcite or dolomite crystals in the parent rock, slightly metamophosed limestones have textures that look slightly blurred compared to a fresh limestone. Such marbles are commonly used as floor tiles and facing stone in public buildings. With higher degrees of metamorphism, the large intergrown calcite and dolomite crystals are more obvious. Such samples are typically passed around in labs.

This is a typical banded marble used as building stone. The banding comes from impurities in the original limestone (such as clay minerals) and is NOT due to any alignment of the calcite crystals. Hence, this banding does not count as foliation. Click here for a super-close-up.
Here is a white marble. The close-up image makes the crystalline texture obvious. Such a sample could also be gypsum or halite, for example. A quick hardness test plus a drop of acid will quickly help you figure out that this is made of calcite and is hence a marble.
Here is a red marble. Again, the close-up makes the crystalline texture obvious. Sylvite, which is KCl, looks very similar to this. A quick lick would tell you whether this is sylvite. (Sylvite tastes like slightly bitter salt. It is the main ingredient in low-sodium salt substitutes.)
This is an outcrop of dolomitic marble in Mosaic Canyon in Death Valley, CA. In close-up view, you can see that the boundaries between beds look slightly blurred. This marble saw a fair amount of stretching, as evidenced by the lens-shaped pods. These are parts of the original dolostone that for some reason were not as easily stretched and thinned.
The light-colored half of the image is all marble, the dark half is an igneous intrusion that through contact metamorphism made the marble from limestone. Photo by Norris W. Jones
This is a close-up of the marble from the quarry above. In the close-up image you can almost see the crystalline texture of the marble. Photo by Norris W. Jones

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