How to Identify Volcano Pebbles and Stones

Appearance can indicate a volcanic rock. Volcanic rocks sometimes contain a chalky and shiny appearance from minerals such as plagioclase and quartz, according to "Igneous Rocks". One example is diabase, which is a dark green or black stone with white crystals. A similar rock called gabbro is also dark green or black with white crystals. These rocks often feel like they are grainy, and both turn brown over time.

Volcanic rocks can also be very porous and have a rough texture. Pumice is an example. This rock is usually white, yellow, gray or red. Pumice is made from the top layer of lava, the froth. This stone is unique in that it is usually light enough to float in water. Pumice is also frequently used in gardens because although it usually has rough edges, it can be easily shaped and crushed and is even mixed sometimes in dirt.

Basalt is one of the most common volcanic rocks.It is gray or black and is made up of fine crystals. Basalt is hard, difficult to break and can contain gas bubbles. Basalt makes up most of the world's crush, according to "Igneous Rocks." Another frequently found igneous rock is granite. Granite is usually brown, yellow, reddish or gray. It contains a large percentage of quartz and is very hard so it can look glassy. Granite is often used to make table tops or counter tops.

Obsidian is a glassy black rock that often includes crystals that look like snowflakes. Obsidian is delicate and breaks like glass. It can be sharp or smoothed around the edges like it was scooped with a clam shell. It can be easily shaped and was used by Native Americans to make arrowheads. Scoria is usually called lava rock and contains gas bubbles. This rock is brown or gray and is often used in landscaping.