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Lab Exercise 3 - Gems in Igneous Rocks

Identifying the settings where gems occur in igneous rocks.

The instructor will give you... a list of numbers corresponding to the labeled minerals in our trays of mineral fragments you will test from your trays. Most of these are broken pieces not crystals.

Record your observations:
After a discussion about igneous rocks, you will be given a box of rocks and asked to identify their origin
and name them based on a brief description. You should learn to recognize these rocks for an
exam. Your instructor will emphasize certain rocks and will help you to correctly identify the samples.

Step A. List of rock numbers for you to learn.

Rock Numbers

Lab Exercise Step A:

Look at the texture and Composition (color).

Texture types of igneous rocks

You should locate, 1) glassy texture, 2) fine-grained (aphanitic) texture, and 3) coarse texture (phaneritic), 4) pegmatitic texture which has crystals larger than about an inch in diameter, 5) the bubbly or vesicular texture, and 6) lastly pyroclastic texture made of broken pieces of rock, such as pumice, and ash mixed together. Use the characteristics described below.

Glassy texture
–overall sheen is glassy, often transparent on the thin edges, has a conchoidal fracture pattern on some surfaces.

Aphanitic texture
–overall a flat or dull appearance, typically a uniform color, crystals are microscopic (not visible to the naked eye).

Phaneritic texture–may have several colors (different visible minerals), sparkles when moved in the light.

Pegmatite texture
–too coarse to be in boxes, there is a side table example. Looks like granite.

Vesicular texture–has openings like Swiss cheese, or cotton candy. The gray rock, pumice (AKA: the bathroom stone) may float on water. The darker rock scoria will not float.

Pyroclastic texture–broken pieces of rocks (lava fragments, pumice, etc.) fused together by heat. The pieces may be different colors. Sometimes layered (usually lighter pumice and darker layers of glass).Often this rock is difficult to recognize in small specimens.

Porphyritic texture-rock displaying individual crystals scattered or "floating" in a fine-grained groundmass. The large crystals formed slowly at depth before being carried toward the surface where the remaining molten material cooled quickly (preventing large crystals from forming in the groundmass).

Step B. List the numbers of the rocks in the table below.
Sample ID


Texture and composition

Step 1. First recognize that texture is along each row, Use column 1 (down) to put in all textures in this table to figure out what you have in your tray. Put the sample numbers in the first column, such as coarse, fine, glassy, etc. (note pegmatite is in with granite). Once you have the textures from the box, check with the instructor because some samples must be on the side table because we may not have enough samples of them.

Step 2. Composition is a result of the minerals present and often is reflected by the color of the rock as listed below. Four color groups are used, felsic (light colors), intermediate (file cabinet gray or salt and pepper), mafic (dark colors such as red, dark brown, or black), and ultramafic (having dark green colors).

As you will see in the chart of igneous classification on the next page’s chart, you need texture and composition to identify an igneous rock. However, composition is basically the same as color (composition = color)!

The amount of dark minerals is all you need to classify color (= composition)!

Remember from earlier that dark colored minerals are: Red, Brown, Dark Gray, Black, and Green, look for the percentages of these colors in the rock. These dark silicate minerals are called the ferromagnesian silicates. So the ferromagnesian silicates can be used to classify the composition of the ordinary igneous rocks! There are a few oddball igneous rocks that have no ferromagnesians. Remember that significant green is ultramafic!

We then classify the igneous rocks base on composition (color) as:
Felsic–less than 25% dark minerals; quartz is often present (usually pink and white colors).
Intermediate–more than 25% and less than 45% dark minerals (usually an intermediate gray).
Mafic–more than 45% and less than 85% dark mineral (usually dark gray or black, some brown).
Ultramafic–more than 85% dark mineral, olivine often present (usually green with some red or brown.)

1) put the rock numbers in the texture box on the left (check to see if there are side table rocks)
2) move the numbers over to the correct compositional box. Go over these with your instructor
3) later record the number and its description on the next two pages of charts

Igneous Rocks Classification Table


Felsic Light colored Igneous rocks mainly pink, white, and gray less than 25% dark minerals (see chart) Intermediate
Gray colored Rocks mainly gray, from 25-45% dark minerals, quartz uncommon

Dark colored igneous rocks mainly dark green, black, or brown >45% dark minerals but less than 85%
Very dark colored rocks containing almost all iron- and magnesium-rich minerals, greater than 85%

Plutonic rocks with coarse texture

Coarse grained
(visible minerals)
Granite if grains are less than 2.5 cm.

Pegmatite if grains are larger than 2.5 cm

Usually a dark gray with black specks ("salt & pepper" look)

Black, but crushed grains on broken surfaces may appear much lighter

Green, usually grains are sugary looking.

Volcanic rocks with fine-grained texture or glassy texture

Fine grained
Rhyolite–usually pink or whitish gray. May have small dark minerals Andesite–usually a medium gray often with splintery dark minerals (shiny bits) Basalt–usually black or dark reddish brown with even texture no rocks in this category
Glassy texture Obsidian–a natural glass used in arrow heads uncommon in this category no rocks in this category
Glassy, bubbly texture Pumice–the bathroom stone, floats on water uncommon in this category no rocks in this category
Non-glassy, bubbly texture no rocks in this category uncommon in this category Scoria–dark brown, red, or black may have olivine grains no rocks in this category
Pyroclastics texture Tuff–a volcanic glass and layered rock. Layers are subtle look at all sides carefully Kimberlite–this is the rock that contains diamonds. Fragments of many rocks possible. May have some calcite, so may fizz with acid!

Some Final Considerations of the Composition of Igneous Rocks.

As we will discuss, felsic rocks are much more common on the continents. Because they have reduced amounts of ferromagnesian (iron and magnesium) or dark minerals, they are low density and float on the denser mafic and ultramafic rocks. Density differences are one reason for plate tectonics. A differentiation by density makes the continents behave differently than the ocean and underlying mantle rocks. The dark minerals (ferromagnesian minerals) have higher melting points and thus sink, as the rocks sink the non-ferromagnesian or light minerals melt and the created magma rises back up. So the ferromagnesians which are still solid and dense sink back into the mantle and the lighter elements (essentially minerals) in the magmas rise.

Plate tectonics will be discussed in lecture and is important for the overturn of materials between the crust and underlying mantle and explains the movement of continents (continental drift) over geologic time. The churning caused by plate tectonics cause less denser (felsic; lighter) minerals to rise up and form the continental crust. Thus plate tectonics does ultimately control where rocks and gem minerals will be present. It also is an encompassing theory that explains the origins or earthquakes, mountain belts, and the chemical evolution of the earth, but this is a topic for physical geology and will not be discussed in detail here. However, geoscientists have the best grasp of the reason for gem, precious metal and all other economic deposits and this course might be a gateway to future studies of the earth!
Plate Tectonics Plate Tectonics Plate tectonic map of the world Figure 5. The plates shown here make up the earth’s outer surface and move on a plastic layer below. Plate boundaries, like the edges of broke ice on a lake, see most of the action.
Plate tectonic model Plate tectonic model (USGS) World map of tectonic plates  
Figures 6A & B show two important boundaries of plate tectonics. Figure 6A shows a ridge in the middle of the Atlantic Ocean where new igneous rock is rising to create two new plates moving the USA away from Europe. Figure 6B shows where plates go under continents after growing from a midocean ridges and sink and melt. The regions near to these sinking (or Subducting plates) experience igneous activity (rocks melt) including volcanic eruptions. Living near a plate boundary has its negatives, such a volcanoes and earthquakes, but much mineral wealth is generated by the processes, including gold and gemstones.

Once you have completed your tray identifications, each table will be assigned a side table sample. You will identify it and look up some of its characteristics and then use these characteristics and terminology to present your findings to the class using guide questions. Once you have completed your tray identifications, each table will be assigned a side table sample. You will identify it and look up some of its characteristics and then use these characteristics and terminology to present your findings to the class using guide questions. You do not have to use all the guide questions and you can make your own questions and answer them instead if you so choose!
Your side
table samples
Kimberlite Pillow lava Pegmatite Volcanic
Vein quartz
with gold

2) Pillow lava and related “geodes” (There are sedimentary geodes; these are not!)

Pillow lava is a rock created when lavas, typically basaltic lavas, flow underwater, such as in a lake or the ocean. Small pillows or blocks of lava pile up (Figures 2) , but there are typically spaces between the pillows that allow later mineralization to occur in between the pillows. These spaces can be filled with many minerals, but a typical infilling is quartz, such as amethyst and zeolite minerals such as natrolite and stilbite. The figure (Figure 8) above is from Paterson, New Jersey, less than 25 miles from New York City (Sinkankas, 1974).

Amethyst is the purple form of quartz, usually forming good crystals. These grow into a hollow between the pillows. Pillow lavas have such “openings” as do “geodes.” The term pillow has to do with the shape of the lava, not the holes or openings between them, but other lavas flow like sheets and don’t have openings of this size for minerals to grow in.

Pillow lavas forming on a lake bed or seafloor (undersea volcano)
Pillow lavas form where lava erupts underwater (oceans or lakes).
Pillows form where the lava on the surface chills quickly but the lava
inside, under pressure) continues to flow or expand from gasses
trapped inside. Eventually the flowing lava ruptures and a new pillow
forms. Hollow areas between pillows or gas bubbles within the core
of pillows are places where minerals can precipitate, filling voids with
crystals (similar to geodes).

Consider the following in making your report.

A) Pillow lava–why this name?

B) What rock type is the lava?

C) Can you find a “Youtube” video on pillow lava.

D) Name or describe some locations and say a little bit about them (hint: Hawaii, California, New Jersey, Brazil). In Brazil, amethyst cathedrals really are the result of pillow lava flows, let’s find some pictures and discuss the evidence.

E) Some of the minerals associated with pillow lavas include, quartz, zeolite minerals, and calcite. Let’s see if we can find something out about these minerals and their local availability,such as in New Jersey.

Crystal-filled cavity in pillow basalt
Click on small images for a larger view. Vug in pillow basalt from Patterson, NJ  
Pillow basalt in the Marin Headlands, San Francisco, CA
Pillow basalt exposed in the Marin Headlands (Golden Gate National Recreation Area), San Francisco, CA Vug from inside a lava pillow filled with crystals of prehnite dahlite, and other minerals from Patterson, NJ.   "A typical angular cavity in pillow basalt from the Prospect Park Quarry (Vandermade), Paterson, New Jersey. This cavity formed at the intersection of several rounded masses or 'pillows' of basalt. The basalt remaining on the specimen is greenish in color, much altered and very brittle." Modified from Sinkankas (1974).
3) Quartz Veins

Be Careful! Warning, the valuable mineral on the quartz veins is very soft and could be scraped off with your fingernail! So what is it?

Well before you answer this question, there are things to consider. People have gone prospecting for this valuable mineral for thousand of years, and though it mainly is found in streams/rivers, there are primary deposits of this metal found in some igneous provinces. This precious mineral has an affinity for the watery solutions found circulating above an igneous intrusion. Some people would say it is not truly igneous since it is not from a direct magma, but precious metal still has an igneous association.

A) Your samples was part of a larger vein of quartz. Describe a vein of quartz and find a picture on the Internet that we can project.
B) Where in the USA might this valuable mineral be found? Mention a place or two (not a stream deposit, but a vein area).
C) What are some properties of the materials involved? Valuable versus what is called the gangue mineral? Notice that in the illustration above dark areas represent open space. What might minerals that grow into that space look like compared to if all the space was filled?
D) What is a nugget compared to crystallized material from primary deposits?
E) Borrow a microscope and set it up so that people can look at your specimens. Could this be a fake? Give some opinions after you have looked at it with the microscope.
Quartz vein in sandstone
Quartz vein in sandstone. Veins typically follow pre-existing fractures or small fault zones.
quartz veins in metasandstone quartz crystals travertine and quartz amethyst Click on small images for a larger view.
Quartz-filled veins in sandstone, North Cascades National Park, Washington Quartz crystals from an open cavity from a mineralized quartz vein from the Santa Cruz Mountains, CA Banded layers of quartz crystals and travertine (calcite) from a mineralized vein from the Santa Cruz Mountains, CA

Amethyst crystals (a purple gem variety of quartz) derived from vein deposits from Minas Gerais, Brazil

4) Pegmatite

First look up or find a description of pegmatite. It is a lot like granite. Your samples have blue/blue-green minerals in it that are sometime valuable. These are not as clear as some gem material. Yours may have fractures.

A) Can you recognize some minerals in it? Look up pictures and try and figure out what the blue mineral is. What the pink mineral is. What the gray mineral with no cleavage is. What the platy mineral with one cleavage is. The blue/bluegreen mineral was not in your mineral box, but the others were.

B) Ask your instructor for last weeks tray of minerals and identify the gray mineral, flaky mineral, and pink mineral in your tray samples. These same mineral are in the pegmatite.

C) Discuss with the group your definition of pegmatite and what kind of gems you might find in a pegmatite.

D) Can we find pictures on "Google Images" or a film of about pegmatite on “Youtube,” etc? (May not be easy!).

E) What is a "gem pocket?"
Click on small images for a larger view. gem pocket  
Schorl, quartz, and feldspar crystals in pegmatite from the Black Hills of South Dakota
Example of a pegmatite with black schorl crystals in large quartz and feldspar crystal masses.
tourmaline and quartz
Tourmaline (pink and green) in quartz pegmatite from San Diego County, California
hornblende gabbro pegmatite from the Aromas Quarry in Santa Cruz County, CA
Hornblende gabbro pegmatite from Aromas Quarry, Santa Cruz County, California


Sinkankas, J., 1974, Prospecting for Gemstones and Minerals, 2nd edition, New York, Van Nostrand Reinhold, 350 p.

"A Cross-section of a gem-bearing pocket in the Himalaya pegmatite at Mesa Grande, San Diego County, California. Although pocks of this type seldom exceed six or eight inches in depth, the extend considerable distances laterally and therefore provide enormous quantities of gem and specimens of tourmaline as well as other desirable specimens (Sinkankas, 1974).
5) Peridote from Arizona and Hawaii

Peridote is a green gem quality olivine (as a mineral it is called olivine) that does not form on the earth’s surface or crust, but comes from the mantle, sometimes starting as deep as 75 Km below the earth’s surface. Olivine and high pressure structural variants constitute over 50% of the Earth's upper mantle, and thus olivine is one of the Earth's most common minerals. The rise of these deep (mantle formed) minerals is unusual and so peridote is a rare gem. Both diamond and peridote come from the mantle.

A) Explain, in simple terms, what the mantle is? Give or find an concentric layers and the mantle should be a focus of your picture. We may project this.

B) Find something on the internet about mantle sources for minerals like peridote. Here is one link: for “Gems formed in the mantle" on this page.

C) Your igneous rock sample is one that we studied today. What is it called? Why is it volcanic or plutonic (is there evidence in the rock that supports your statement)? What is its classification in terms of texture? What is its classification in terms composition (but remember the peridote is not a phenocryst, but a xenolith)? Being that the peridote is a xenolith, is this rock truly ultramafic? Just consider the matrix or ground mass and not the xenoliths.

D) What is a flood basalt? Find some gems that come from these? Name a few locations.
small olivine crustals in xenolith in basalt lava from Halualai Volcano, Kona, Hawaii
Green rock consists of small olivine crystals (the gem is called peridote) occurs in xenoliths in a basaltic lava flow from Halualai Volcano near Kona, Hawaii

Evaluation Questions for Igneous Rocks

1) Would a denser rock (one that has denser minerals) generally lie toward one side of the classification of rocks (Igneous Rocks Classification Table)? Think about composition.

2) Could the phenocrysts in a rock be potential gemstones? Explain.

3) Would finding a particular igneous rock type get you excited about potentially becoming rich? Explain.

4) Lavas tend to have what type of grain size?___________________. Is the grain size a negative for finding gems in A) some instances, B) all instances, C) let me research that--What gems are found in lavas?

5) What is the general origin of obsidian? Can you find two present/past uses of obsidian that make it valuable?

6) What attribute sets pegmatite aside from other rocks? What gems might one hope to find in it?

7) Locally pegmatites have produced gems. Can you find a local example of a gem producing pegmatite in New York, Connecticut, or Pennsylvania? (Hint try: “Gemstone Occurrences in Connecticut”; see if that includes pegmatites). What was produced?

8) Why might it be easier to get a gemstone crystal (specimen that looks nice) out of a pegmatite than out of a granite or basalt?

9) A very important mineral in igneous rock classification is a green mineral. As we learned last time this mineral is considered a dark-colored, or ferromagnesian silicate. What is this mineral? Towards which side of the chart is it found (left or right)?

10) Quartz is almost as important as the green mineral mentioned above. What side of the classification chart of igneous rocks does it tend to be on? (Felsic or Ultramafic)

11) Pillow lavas form when lava __________________________ and tend to have _______________________ that minerals can grow in. See if you can fill this in.

12) Plate tectonics has moved things around. Continents have drifted. Continents have been torn asunder (apart by rifting). Think about it. Could gems prospecting and an understanding of plate tectonics have a potential to help/explain each other.

13) What about volcanoes and plate tectonics? Where is “The Ring of Fire?” Could volcanoes help create gemstones?

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