Do you know what is the heaviest rock?  I thought I knew the answer, but found to my surprise the answer to that question was not granite.

Recently my kids and I found some pumice, which is a super lightweight rock.  That got my son’s little brain thinking.  He asked me what was the heaviest rock?    I wasn’t sure at the time and was surprised at the answer I found when I researched it.  The answer is a little bit complicated, but here goes my explanation.

Rocks Are Comprised of Minerals

First to understand why some rocks are heavier than others we need to know what rocks are made up of.  A rock is a solid structure that is made up of one or many minerals.

REMINDER – There are three main kinds of rocks in the world: sedimentary, metamorphic, and igneous.  Here is a post about the rock cycle that explains how these rocks get made and change into other forms.

What Are Minerals?

A mineral is a naturally occurring solid that has a specific chemical composition and crystal structure.  In other words, a mineral is the smallest solid structure that combines to build rocks.  Rocks are made up of different minerals.

There are more than 500 known minerals in the world, but only about 100 of them are common.  In fact just three minerals – quartz, feldspar, and olivine are by far the most common and make up about 90% of the rocks at the earth’s surface.

The Heaviest Rocks Are Made of Dense Minerals

The heaviest rocks in the world are those that are made up of the heaviest or densest minerals.  Geologists measure how dense a rock is by comparing it with water.  Water weighs 1 gram per cubic centimeter.  Every other mineral is compared to that with denser or heavier ones having high numbers and lighter or less dense ones lower numbers.

In general, the metallic minerals have higher densities and non-metallic minerals have lower densities.  Most minerals are in the range of 2 – 4, but the most dense minerals are gold at 19.32, iridium at 22.42, and platinum at 21.45.

Think of a mineral as a lego piece.  Rocks are lego structures made up of hundreds or thousands of individual lego pieces.  The different kinds of lego pieces do not weigh the same.  Some of them are very dense, while others are lighter.  The overall weight of the lego structure depends on what individual lego pieces were used to build it.

What Is The Heaviest Rock?

How heavy or dense a rock is depends on the kinds of minerals that it is made up of.  In general, sedimentary rocks tend to be less dense than metamorphic or igneous rocks.

I always thought that granite would be one of the heaviest rocks.  From rock climbing I know that granite is a very strong rock that does not easily erode such as sandstone.  To my surprise I found that granite has a density of 2.6 – 2.7 compared to sandstone at 2.2 – 2.7.

The heaviest rocks would be those that are made up of dense, metallic minerals.  Two of the heaviest or densest rocks are peridotite or gabbro.  They each have a density of between 3.0 to 3.4 grams per cubic centimeter.  Interestingly, peridotite are the rocks that naturally occurring diamonds are found in.

Note – even though the heaviest minerals are gold, iridium, and platinum they are not the heaviest rocks.  They are not in large enough quantities within a rock to make the overall rock that much heavier than other rocks.  Also, a rock would not be made up of just those minerals, but would be combined with many others.