Rock Cycle - Rocks (2)

  • Observing the three types of rocks.
  • Comparing rocks.
  • flat
  • grain
  • light (weight)
  • shiny
  • smooth

Students describe rock samples.

Mudstone with fossils


The name of a rock reflects certain characteristics. For example, obsidian will resemble glass and scoria will usually be dark red with holes. Rock names also refer to a texture. For example granite will have interlocking minerals and sandstone will have a gritty, sandy feel.

Young children need to experience these characteristics before they can internalize the name of a rock. They need to describe and compare the characteristics, as they learn the rock’s name. Just knowing a name of a rock is not enough. In this lab, students will use their observational skills to classify rocks. Rather than using a formal classification, the emphasis is on students developing their own criteria for rock classification.

  1. Review the three main groups of rocks with students that they learned from the Pre Lab.  Be sure to tell them that there are many different types of rocks within each of the three groups. Brainstorm possible terms for describing rocks with the students.
  2. Explain that identification and classification of an individual rock specimen is based on a variety of characteristics and criteria, and that they will start to learn some of these in this lab.
  3. Give each group of students a piece of red, yellow, and black paper. Write red = igneous, yellow = sedimentary, and black = metamorphic on the board. Pass out the rock sets. Go through each of the rocks to make sure the students put the correct rock on the right color. Use the information on each rock as outlined below. You may want to give the students some clues of each rock, and then when you review the lab, you might want to add more information.


OBSIDIAN - Also known as volcanic glass. Most children recognize obsidian as the rock that many Indians used to make arrowheads. The Indians chose obsidian for the same reasons that a geologist can recognize it. It is very hard, but more importantly it breaks into sharp edges that easily cut through many materials. Note that broken obsidian looks like broken glass. Obsidian occurs in almost any color, depending on what trace elements are present in it. Black and brown obsidian are most common. Obsidian is an amorphous solid; that, it is a solid rock composed of silicon dioxide, but this material lacks crystalline structures. It is one of very few exceptions to the rule that rocks are made of minerals.

The obsidian that is in your kit comes from volcanoes near Clear Lake, California. Obsidian is formed when lava is cooled very quickly; it freezes before crystals can form. Have your students try to determine which part of a lava flow will cool quickly enough to form obsidian (answer - the outer surface or "skin" of the flow).

PUMICE - Students will immediately notice that pumice is spongy or "full of holes" or vesciular. This characteristic makes pumice extremely lightweight; it even floats in water (you may wish to show this to your students). It is commonly light gray to blackish-gray in color. It is easily broken and has sharp edges. Like obsidian, pumice is volcanic glass; it thus looks glassy (especially with a magnifying glass) and lacks visible minerals.

Pumice forms during eruptions of magma containing large quantities of gasses, such as water vapor, sulfur dioxide, and carbon dioxide. The gas "froths" the magma as it erupts, forming bubbles. This is physically analogous to opening a soda can; carbon dioxide bubbles form in the drink as the can is opened. Like obsidian, the magma then cools quickly, preserving the bubble shapes. The gas often escapes, leaving numerous holes in the pumice. Pumice is used as an ornamental building stone. "Pumice rock" is also sold in beauty stores for cleaning dead skin cells from areas like feet or elbows.

SCORIA - Scoria is composed of pieces of volcanic glass and preexisting rock fragments that became incorporated into the magma as it erupted.  Many scoria have fragments of basalt included hence if can look like a red vesicular basalt.  The volcanic glass looks similar to pumice but denser. Scoria is reddish in color, because contains more iron than pumice. Scoria lacks large visible minerals; small ones may be it visible with a magnifying glass. Scoria is often sold as "lava rock" for use as a landscaping material.

GRANITE - Granite is composed of visible minerals, most commonly quartz, mica and feldspar. Quartz looks clear and glassy, mica is black and flaky, and the feldspars (commonly two or more different types are present) are either pale pink/orange or white in color. The relatively large size of the minerals indicates that the magma that formed the granite cooled slowly. This took place deep inside the earth, not on the surface, like pumice or scoria; it is a plutonic rock. Ask your students if they think granite is made of the same minerals as basalt (no, they cooled differently and came from a different "mother" magma). It may help to have them imagine that the minerals in the granite were tiny; would this make them dark?, (No, they would still be light colored). This indicates that rocks composed of different minerals likely have different magma "mothers." Try using the analogy that rocks are like people, no two are the same! Granite is used as ornamental and building stone.


CONGLOMERATE - Conglomerate consists of pebbles, gravel, sand, and boulders that have been cemented together to make a solid rock. These materials were mixed naturally in rivers or in some parts of oceans and lakes. Any type of preexisting rock can become part of a conglomerate.

To explain cementation, try telling students that Mother Nature has a cement that she sometimes pours onto the beaches of lakes, oceans, and rivers. When it hardens, it becomes conglomerate, if the pieces are big, or sandstone, if they are small. In reality, the two most common cementing substances are natural solutions of calcium carbonate and silica dioxide. Crystals of calcite and quartz, respectively, precipitate from these solutions in the spaces between grains, cementing the rock together.

SANDSTONE - The gritty feel of the surface of sandstone hints that this rock was once sand that has been cemented together. Sandstones have quite varied compositions; some are composed entirely of quartz, and others are mixtures of rocks, crystals and fossils. Almost any combination is possible. Sandstones thus come in a wide array of colors. By definition, the grains in a sandstone are "sand-sized"; most students will recognize this if you demonstrate "sand size" by showing them a bag of sand.

SHALE - Shale is composed of very small particles of mud, which have been compacted and cemented together. Individual mud grains are very small; they will rarely be visible. Shales are quite variable in color.

CHERT - Chert can range in color from white to red to brown in color and is largely composed of very small quartz crystals. The red color comes from trace amounts of iron, and brownish tinges can be caused by the presence of organic matter. Chert is very hard. Chert was also used by Indians (the variety called flint) for making tools. Chert forms from the skeletons of microscopic one-celled protozoa called radiolarians. These are sometimes preserved in the rock, but can only be seen with a microscope. Chert forms on the ocean floor, where the skeletons of these organisms are deposited after they die.

COAL - Coal is a sedimentary rock composed of the accumulation of vegetable matter that  has been consolidated between other rock strata to form coal seams.  Coal has several forms depending on the  effects of microbial action, pressure and heat over a considerable time period.  Anthracite is the most compressed and hard form; bituminous is not as hard; and lignite still has vegetable matter present. 


MARBLE - marble is composed exclusively of large commonly visible crystals of calcite. The gray/white bands in some of the samples are due to impurities within the calcite. Marble actually comes in a variety of colors, including black, gray, white, and pink. Marble, like all rocks that have calcite in them, fizz if you put a weak acid on it (usually 10% solution of hydrochloric acid). Marble forms when a rock containing calcite in it (such as limestone) was put under high temperature and pressure conditions. Marble has been used throughout history because it is easy to break and to carve. Some marble (especially in Italy) is noted for its smooth, small crystals that make it excellent for statues. Many of the statues of Michelangelo were made from marble. Marble is also used as an ornamental building stone. If you live near or in a city, have your students try to find buildings made of marble. If you are in an old school, some of the bathroom stalls or floors may be made of marble.

SERPENTINITE - Serpentinite has a smooth, soapy feel, a green mottled color, and a somewhat flaky texture. It is composed mainly of the mineral serpentine. Serpentinite is so named because of its mottled color, which resembles the back of a sea-serpent. The geologic origin of serpentinite is still debated, but many scientists agree that it formed from a rock like basalt that was put under high temperature and pressure. Serpentinite is the state rock of California. Serpentinite is used for carving and as an ornamental building stone.

SCHIST - Schist is composed of visible minerals, mostly micas. Schists form under moderately high pressure conditions; this causes the naturally platy mica crystals to line up, giving the rock a platy look. This is a good example for illustrating the characteristic "squished" look of metamorphic rocks to your students. Have them imagine that a heavy Mother Nature sat on some rocks - look at what she did!

[Back to Rock Cycle Grid]   [Back to Rocks (2)]