How it's Made: Marbles

From TechEBlog:

If you've always wondered how marbles were made, then check out this interesting "How it's Made" segment. Pure white marble is the result of metamorphism of very pure limestones. The characteristic swirls and veins of many colored marble varieties are usually due to various mineral impurities such as clay, silt, sand, iron oxides, or chert which were originally present as grains or layers in the limestone

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