Learn about the two main factors that determine stone’s characteristics.

Origin – How was it made?

• Igneous
• Sedimentary
• Metamorphic

Composition – What is it made of?

• Silicates
• Calcium Carbonates

I. Origin

Igneous Stone – "Born of Fire"

Granite comes from igneous rocks, formed slowly, as it cooled deep underground. Their minerals look like small flecks typically spread consistently throughout stone. Some other types have veining (linear waves) like marble. They are hard (cannot be scratched by steel) and dense.

Sedimentary Stone – "Cementing of Grains"

Sandstone and limestone would fall into this category. These stones are formed through the compacting of grains or pieces of any kind of existing rock material. These existing rocks may have been weathered, transported, deposited and then cemented over millions of years by the movement of the earth’s tectonic plates. From the formation of the continents to an earthquake or volcanic eruption, all of these events have helped form this stone. Sedimentary stones may even contain fossils or other distinct features formed at the time of deposition.

Metamorphic Stone – "Changed in Structure"

Marble and slate are metamorphic stones. They were formed at extremely high pressures and temperatures below melting. The presence of swirls, linear patterns or banding is a key characteristic. Slate is a fine grained, metamorphic rock, which cleaves in flat, almost smooth pieces. Marble is a metamorphic limestone that loses the fossils and other features during the recrystallization.

II. Mineral Composition

Silicates

These stones are made mostly of quartz-like particles called silica. They are very hard, durable and generally acid resistant. Examples: granite, sandstone, slate and quartzite.

Calcium Carbonates

The minerals in these stones were formed under pressure over millions of years from the bodies of tiny fossilized creatures. These stones are softer, less durable than silicates and acid sensitive. Examples: limestone, marble and travertine.