for school purposes

1. ALL ABOUT ROCKS
DEFINITION AND CHARACTERISTICS

2. WHAT IS ROCK
To geologists, a rock is a natural substance
composed of solid crystals of different
minerals that have been fused together
into a solid lump. The minerals may or
may not have been formed at the same
time. What matters is that natural
processes glued them all together.

3. MAIN CHARACTERISTICS OF ROCKS
1. Hardness
Hardness is perhaps the most important characteristic of a rock
because it is a measure of a rock's resistance to abrasion,
scratching, and crushing. Hardness is also a function of a rock's
mineral composition and texture.
TheMohs Scale of Hardness is the most commonly used way to
measure hardness. The scale was developed in 1812 by German
mineralogist Friedrich Mohs and is based on the ability of one
material to scratch another. The scale is logarithmic, with each
increase in hardness representing a 10-fold increase in
resistance to scratching.

4. 2. Luster
Luster is the way a rock's surface reflects light and is an important
characteristic for identifying a rock. Rocks can have two types of luster:
metallic and non-metallic.
• Metallic luster is shiny and looks like metal. Non-metallic luster is
duller and can range from being pearly to looking like glass.
• Most rocks have a non-metallic luster, but some, like gold and
silver,have a metallic luster.

5. 3. Color
• Though many rocks are some shade of gray, some rocks can be
very colorful. The color of a rock is determined by its chemical
composition. For example, the green in some rocks comes from
the presence of minerals such as chlorite, while the red in other
rocks comes from the presence of minerals such as hematite.
• Rocks can also be a combination of colors, while some may even
change color when exposed to the air or when heated.
• Color is commonly used in conjunction with other characteristics,
such as hardness, to help identify a rock.

6. 4. Streak
The streak of a rock is its color in powder form. It
is obtained by scratching the rock on an
unglazed porcelain plate and rubbing the
resulting powder on a white tile. The color of the
powder is then compared to a chart of streak
colors.

7. • 5. Cleavage
• A rock's cleavage is the way it breaks when struck. Some rocks
have a well-defined cleavage, while others do not.
• Cleavage is determined by the arrangement of atoms in a rock's
crystalline structure. When a rock has a well-defined cleavage, it
means that its atoms are arranged in sheets that can easily slide
past each other.
• Rocks with a well-defined cleavage will break along these sheets,
resulting in smooth, flat surfaces. Rocks without a well-defined
cleavage will break in an irregular pattern.