Identifying Rocks – L


We can identify rocks and minerals by looking at different characteristics.  Some of the most common characteristics to look at are color, luster, streak, cleavage, hardness and specific gravity.  Let’s look at these characteristics a little closer.


Color can vary greatly depending on the chemicals in the rocks and minerals.  However, situations like weathering can cause changes in the color.  Color by itself is not the best way to identify a rock or mineral.


Luster refers to the how the surface reflects light.  Some words used to describe this characteristic include metallic, non-metallic, brilliant, dull, pearly, or glassy.


Streak is the characteristic that describes the color of a line made if you rub the rock or mineral across a rough surface.


Cleavage is the characteristic that describes the pattern or the way a rock or mineral breaks.  Some will break in sheets.  Some break into cubes.  Some will break leaving jagged edges.


Specific gravity refers to an object’s weight compared to the weight of an equal volume of water.  This characteristic describes the density of the rock or mineral.


This characteristic refers to how easy it is to scratch a rock/mineral (and what it can scratch).  There is a scale called the Moh’s Hardness scale that shows how to group rocks or minerals by this characteristic.

Moh’s Hardness Scale


Example Mineral


1 Talc Easily scratched, even by a fingernail.
2 Gypsum Can be scratched with a fingernail
3 Calcite Can be scratched by copper penny
4 Fluorite Easily scratched by steel knife.
5 Apatite Can be scratched by steel knife
6 Feldspar Cannot be scratched easily by steel knife.  It scratches glass.
7 Quartz Easily scratches steel and glass.  (This is the hardest commonly found mineral.)
8 Topaz Harder than commonly found minerals.
9 Corundum Can scratch topaz.
10 Diamond Among the hardest of all minerals.