Glassy Rocks Examples (Characteristics and Types)

In this article, you’ll get a list of some commonly known glassy rocks, as well as background information about glassy rocks to better understand them.

Glassy Rocks Examples (Characteristics and Types)


There are many different types of rocks

. Some of the most beautiful are the naturally occurring glassy rocks.

These rocks usually form when magma or lava cools quickly, so mineral crystals do not have time to grow.

This rapid cooling leaves them with a glassy appearance, hence the name. Examples of glassy rocks include:

1. Obsidian

Physical description

Obsidian is one of the most famous types of volcanic glass.

It is usually jet black, but there are red and brown varieties.

Sometimes, it even has bands of other colors like gray, green, or yellow.

Obsidian is slightly harder than window glass and is usually very shiny.

It can be broken off into very sharp edges and polished to be reflective.

Chemical Composition

Obsidian is about 65 to 80% silica and only about 1% water by weight.

The rest of the composition can vary and determines its color.

For example, iron oxide can make the obsidian reddish or brownish in color.

Tiny gas bubbles in the rock can also change the color of its sheen or luster.


Since it forms when surface lava flows are rapidly cooled, obsidian is likely to be found where there are (or at least were) active volcanoes.

These places include volcanic islands like Iceland and the Eolie Islands off the coast of Italy.

It can also be found near mainland volcanoes like Obsidian Cliff in Yellowstone National Park.

Where it is found, it is usually near the surface.


Obsidian has gained fame not only for its appearance but for its utility as well

. Many Mesoamerican peoples used obsidian to make tools and weapons.

In fact, obsidian blades can be sharper than steel ones.

Ancient Greeks also used the stone’s reflective properties to make mirrors. It was also used for jewelry and is still considered a semiprecious stone.

Obsidian is still being used today, both as a blade and as ornamentation.

Many surgeons also use obsidian for scalpels due to how sharp they can be.

Jewelry is also still crafted with obsidian due to its jet black color and highly reflective surface.

2. Tachylyte

Physical Description

Tachylite (also spelled tachylyte) is a type of volcanic glass that is brown in color and translucent when it is thin and darker and shiny when it is thicker.

Besides being clearer and browner, it is very comparable to obsidian, especially when found in large deposits.

Chemical Composition

Unlike obsidian, tachylyte is relatively low in silica, only having 45 to 52% silica.

Most of the rest of the mineral is made of oxygen.

Tachylite forms from molten basalt that has rapidly cooled.


Due to being formed from molten basalt, it is usually found where basaltic lava flows have cooled quickly.

This includes the Hawaii Islands and the Inner Hebrides in Scotland.


Like the rock it is made from, tachylyte is used for building materials.

It is specifically used as an aggregate in the making of concrete.

An aggregate is a filler substance that helps keep the concrete together.

It also affects its ability to retain water and how much it expands in the heat.

3. Perlite

Physical Description

Perlite is also known as pearlstone.

This name gives a pretty good idea of what it looks like.

It is a natural glass that breaks off into pearl-like pieces.

They are usually pale gray or off-white in color and have a waxy luster.

The pearl-like pieces vary in size but are generally more like pebbles than boulders.

It is commonly sold ground up into a white chalky powder for industrial purposes.

Chemical Composition

The chemical composition of perlite is similar to obsidian, but it has a higher percentage of water.

It contains only about 2 to 3% more water, but that is enough to affect the shape and color of the mineral.

Besides the extra water, perlite is mostly silicon.


The main areas where perlite is quarried are the southwestern United States, Greece, and Turkey.

The earliest sources were in New Mexico, Nevada, and California.

The sources in Greece and Turkey were found later, but both nations produce more than the United States.

Other significant deposits are in Hungary, Italy, and the country of Georgia.


As mentioned above, most of the uses for perlite involve it being ground up.

It is commonly used as a lighter-weight substitute for sand in the mixing of plaster and concrete.

Like tachylyte, it can also be used as an aggregate.

It is usually ground up for this purpose as well.

Another everyday use for perlite is as an additive to potting soil.

It helps prevent soil compaction due to its high permeability and low water retention.

Soil compaction is when soil is pushed together, which lessens the number of pores in the ground so water can’t get to the bottom.

This keeps the water from getting to the roots of plants in the soil.

4. Pumice

Physical Description

Pumice is a light-colored and very porous rock that forms during explosive volcanic eruptions.

It is not as shiny as the other rocks on this list and is usually a light brown or off-white color.

Chemical Composition

The chemical composition of pumice can be the same as obsidian.

Any type of igneous rock that is heated up under pressure and then has that pressure quickly released will form into pumice.

In fact, if you heat up obsidian in a crucible, it will form into pumice.

Due to this origin, pumice can have a variety of different compositions.

It is defined by how it forms, not what it was formed with.


Pumice can be found in any place where there was an explosive volcanic eruption.

The most common sites where it is quarried are around the Mediterranean and in the Western United States.


As well as being used as an aggregate when mixing concrete (like perlite), pumice is also used as a decorative stone in landscaping and horticulture.

It is used to wash “stone-washed” jeans and in abrasive facial cleaners as well.

Actually, it is used as an abrasive in cleaning and polishing stones too.


Glassy Rocks are, as a whole, rocks that formed from sudden volcanic activity.

They all share their glasslike structure, whether they were created from lava flows or explosive eruptions.

They come in various appearances, have a variety of different chemical makeups, and have a variety of different uses.

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Glassy Rocks Examples