Neighborhood Rocks                                     Search this Site

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.
bulletHow to recognize dolostone
bulletOther rocks that look like dolostone
bulletSpecial things to look for
bulletWhere dolostone came from
bulletHow dolostone formed
bulletOther names for dolostone
bulletLinks to Web sites about dolostone

How to recognize dolostone

bulletDolostone is mostly light gray in color.
bulletIts crystals are usually small- to medium-sized, and freshly broken surfaces may glitter in sunlight.
bulletYou can scratch dolostone into white powder with a nail or knife.  (See more about the scratch test.)
bulletDolostone 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

bulletLimestone is made of calcite, so it bubbles gently when you put a drop of white vinegar on it. 
White Marble 
bulletMarble is often a brighter white.
bulletMarble is usually made of calcite, so it usually fizzes gently when you put a drop of white vinegar on it.
bulletMarble'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.
bulletChert is harder -- a nail scratches dolostone, but not chert.
bulletChert is smoother -- you can often see small or medium-sized crystals in dolostone, but not in chert.
bulletChert often breaks to form sharp or scalloped edges.
Gray Slag:
bulletSlag usually has lots of rounded bubble holes.
bulletIf you look closely, you'll see that slag is often somewhat glassy in spots (especially inside bubble holes).
bulletSome 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.

How dolostone formed

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:

bulletWe've seen 50-pound bags of dolostone gravel labeled "Arctic Frost" and "Antique White."
bulletWhen dolostone is crushed into gravel and
used for building roads or making concrete, it's often called an aggregate.
bulletMany 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):
bulletGeologists have a catch-all term that includes both dolostone and limestone.  They call them carbonate rocks.)
bulletDolostone is a sedimentary rock.
bulletDolostone 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:
   < >

     It includes a look a Thornton Quarry, which is 
     pretty close to our home:
     < >

There are also fossilized Silurian-age reefs in Norway.
Visit one here:
   < >








Copyright 2001 Eric D. Gyllenhaal                                             Search this Site

Neighborhood Rocks is part of the Salt the Sandbox Web. 
For more information visit the Salt the Sandbox home page.

This page was created on March 12, 2001, and it was last updated on July 27, 2002.