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Dolostone ("DOE-low-stone")

		Dolostone is one of the most common rocks in our neighborhood. People use it as gravel on their paths and driveways. Road builders mix it with concrete and asphalt, and railroads use it for ballast. Dolostone formed in ancient seas, often in coral reefs, and it sometimes contains fossils. To learn more about dolostone, scroll farther down this page.
Here's some dolostone gravel, dug out of a quarry near our home.

This truck is dumping a load of dolostone on our street.

	How to recognize dolostone
	Other rocks that look like dolostone
	Special things to look for
	Where dolostone came from
	How dolostone formed
	Other names for dolostone
	Links to Web sites about dolostone

How to recognize dolostone

	Dolostone is mostly light gray in color.
	Its crystals are usually small- to medium-sized, and freshly broken surfaces may glitter in sunlight.
	You can scratch dolostone into white powder with a nail or knife. (See more about the scratch test.)
	Dolostone is made mostly of the mineral, dolomite. Dolomite won't bubble in vinegar or acid, unless you scratch off some powder and put the vinegar right on the powder.

Other rocks that look like dolostone

Limestone:

Limestone is made of calcite, so it bubbles gently when you put a drop of white vinegar on it.

White Marble:

	Marble is often a brighter white.
	Marble is usually made of calcite, so it usually fizzes gently when you put a drop of white vinegar on it.
	Marble's crystals are usually larger and more consistent in size than dolostone's crystals.

White Chert: Pieces of chalky white chert are often mixed with dolostone gravel.

	Chert is harder -- a nail scratches dolostone, but not chert.
	Chert is smoother -- you can often see small or medium-sized crystals in dolostone, but not in chert.
	Chert often breaks to form sharp or scalloped edges.

Gray Slag:

	Slag usually has lots of rounded bubble holes.
	If you look closely, you'll see that slag is often somewhat glassy in spots (especially inside bubble holes).
	Some spots bubble in vinegar or acid, but most places do not.

Special things to look for

	Look for fossils. The left arrow points to a piece of brachipod fossil. The right arrow points to a small coral fossil.

	Look for masses of crystals. These quartz crystals filled an open space, or vug, within the dolostone rock. (The specimen is about an inch wide.)

	Look for white and gray chert pieces in dolostone gravels. (Chert is harder than dolostone, looks smoother, and breaks to form sharp edges.)

	Look for fools' gold (pyrite or marcasite) crystals. The dark specks on these rocks look like brassy metal under our Intel Play QX3 computer microscope.

Where dolostone came from

	The dolostone in our area was blasted out of huge quarries, like this one near Brookfield, Illinois. See the tiny dump trucks in this picture? Each one carries tons of crushed dolostone that will be used to make and repair roads and parking lots -- or mixed in concrete to build homes and offices.

Our dolostone started out as skeletons of corals,
sponges, and other animals that lived in a shallow
sea that covered our area more than 400 million
years ago. The skeletons accumulated on the sea
floor as huge mounds, called reefs, and as layers
of sand- and mud-sized bits of skeleton.

These deposits could have formed limestone rock,
except that something happened to change their
chemistry. Sometime after burial, the "lime" in the
skeletons (the minerals calcite and aragonite)
changed into a new mineral, called dolomite.

So, instead of limestone, we have dolostone!

Other names for dolostone

We use the scientific name "dolostone" for this rock,
but it is also known by many other names:

	We've seen 50-pound bags of dolostone gravel labeled "Arctic Frost" and "Antique White."
	When dolostone is crushed into gravel and used for building roads or making concrete, it's often called an aggregate.
	Many geologists call this rock dolomite, which is also the name of the mineral of which it is composed. (We like having different names, so it's clear whether we're talking about the mineral or the rock.)

Here are some ways to classify dolostone (by grouping it with similar types of rocks):

	Geologists have a catch-all term that includes both dolostone and limestone. They call them carbonate rocks.)
	Dolostone is a sedimentary rock.
	Dolostone is sometimes classified as a chemical sedimentary rock, because it usually forms when limestone is chemically changed.

Links to Web sites about dolostone

Here's a Web site detailed information about the
types of reefs where our local dolostone formed:
< http://www.mpm.edu/reef/welcome.html >

     It includes a look a Thornton Quarry, which is
     pretty close to our home:
     < http://www.mpm.edu/reef/thornton-front.html >

There are also fossilized Silurian-age reefs in Norway.
Visit one here:
< http://www.toyen.uio.no/palmus/galleri/montre/english/m_silurrev_e.htm >