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Lab Exercise 4 - Gems in Sedimentary Rocks

Purpose: Identifying the settings where gems occur in sediments and sedimentary rocks.



A) Detrital rocks.

For each sample that your instructor assigns make observations and then describe the sample. Your instructor will tell you whether the samples are detrital or chemical.

Detrital rocks (made of particles)  

For the detrital (made of particles) sedimentary rocks the following table will help. Place sample # next to the rock name.
Grains that compose sample and Common characteristics Size of grains Name of detrital rock Place sample # in this box
Angular fragments Often all grains are one color and cement is another color Particles greater than 2 mm Breccia Gravel
Gravel (rounded pebbles) Often our samples have sand grains as well Particles greater than 2 mm Conglomerate
Sand Often white, gray, or tan Feels like sand paper Particles less than 2 mm Sandstone
Clay Gray, green, black, red smooth and chip like shape Thin but wide pieces Particles invisible (clay) Shale
B) Chemical rocks.
Chemical rocks (made of organic particles or precipitates)  

For Chemical rocks and organic rocks use the following table to name them.
Notice that the chemical and organic rocks tend to be rather soft. Mostly the chemical rocks are salts made by a combination of positive and negatively charged ions or molecules combining. The amount that these dissolve is an indication of which will first precipitate as water begins to evaporate. Some limestones are inorganic salts others contain fossils.

Hardness and Characteristics Reaction to acid or other characteristic properties Name Place sample # in this box
Softer than fingernail May be pink, white, or colorless-opaque or transparent none Rock Gypsum
Harder than fingernail Often shows cubes usually gray or colorless. none, but may dissolve Rock salt (should be translucent)
Harder than fingernail Fine, smooth, no fossils bubbles, fine-grained lithographic limestone
Harder than fingernail Broken bits of shell that look like oatmeal bubbles, but acid may sink in rapidly to the holes Coquina (little shell) limestone
Harder than fingernail fossils may be bits of shell or coral bubbles, contains fossils Fossil limestone (organic)
Softer than fingernail black color may have shiny parches, light weight brownish-bbrownish-black, flakeylack, flakey Bituminous coal (organic)

Descriptions of Sedimentary Rocks

Specimen Number detrital–particle size organic–constituents chemical–minerals physical properties rock name


1) Animal fossils

Fossils are both highly prized as collectors items and as evidence of past life and needed for scientific study. As well the shells of some molluscs retain their mother of pearl luster after complete fossilization and are referred to as ammolite, a gemstone. Of course, the fact that fossils were once living in the area where the rocks formed and are constituents of sedimentary rocks indicates the conditions the sedimentary rocks was forming under. Fossil remains can make very attractive display pieces and are highly collectible. Many of the molluscs (clams, mussels, snails, etc.) once had calcium carbonate shells that were pearly. Do you see any pearly material in your examples of limestone? Only rocks going back to about the age of dinosaurs preserve mother of pearl. Older fossils in rocks tend to lose the mother of pearl. Time, dehydration, and deterioration of organic material change the rock and fossils. Ammonites were a type of marine mollusk similar to a squid or nautilus that live today. Their shells could be very large and they could produce mother of pearl. This fossil mother of pearl is mined to make jewelry.

Explain some facts about ammonite.
A) What is it? Give a description. Can you find a picture of the animal to project?
B) Where is it mined?
C) What kind of rocks is it in? Did the animals live in a specific place?
D) Is the material hard enough to make a good gem or is it a little soft and delicate?

2) Plant fossils

Rocks of organic nature include the coals, anthracite and bituminous coals both are accumulations of plant matter that was crushed and slowly lost volatile constituents becoming a carbon rich material. Anthracite has undergone more heating and pressure and many geologists consider it a metamorphic rock. It is used in carving as well as in mourning jewelry, and as a stocking stuffer for naughty children.

Jet is a related type of coal that takes a good polish and is used in the same manner as anthracite.
A) What is Jet? Where does most jet come from? Country of origin?
B) Explain details of what mourning jewelry is. Is it popular today?

C) Are there substitutes for jet these days that may not have been around in the past?
D) Are there some properties of jet worth mentioning? Hardness, density, etc.
3) Amber
Amber is not itself a true fossil, it is fossilized tree resin, but amber can contain fossils and some pieces are highly collectible.
A) What is amber? Explain a little about the origins of amber.
B) Why are fossils in amber usually different than fossils in rock?
C) What are some important fossils found? Why is amber part of Jurassic Park the movie?
D) Explain a little about amber’s properties and what fake amber might be.

4) Pebbles of Sapphire from Montana

Pebbles of sapphire have been found in streams and ancient stream deposits of Montana. Some are of excellent color and clarity. Though sapphires are usually thought to be blue they can be almost any color other than red, which is a ruby. Sapphires from Montana come in many colors. Some fine ones are found in basalt (igneous) flows in Yogo Gulch. Do not discuss these igneous ones, stick to the detrital sources (alluvial/river derived).

A) What is sapphire made of? Chemical composition. Give a brief description including Mohs’ hardness.
B) What is their source in Montana thought to be? Is there a rather famous river involved?
B) Do the samples you have seem very rounded? Based on the rounding have they traveled far? C) Do the crystals/grains seem to have any similarity in shape or size? This might in part be due to their processing, but could also be a geologic property. About how big are the largest in diameter compared to the smallest.

Everyone do this as homework

5) Witwatersrand Gold Ore Sample
This rock came from South Africa. It contains gold, but can be classified as “noseeum” gold.
Sometimes the gold is visible, but mainly one sees larger bits of “fool’s gold” that contains the true gold that is microscopic.
1) Look for the pyrite.
2) Look at the rock texture. Can you tell what rock it is? Name it. It is pervasively cemented.
3) Find out about this “ore” and give a little history. What does ore mean? How old is the ore?
How did the gold get their?
4) More history, who found this gold? When? What famous city is located near it?
5) How deep are the mines? Try and find comparison to average mines.
6) How much gold is/has been found? Altogether? Per year? Per ton?
7) There appears to be other precious/valuable material in this rocks as well. What?
8) How important a deposit is this? Was it world changing?

Evaluation Questions for Sedimentary Rocks
1) Detrital sedimentary rocks are more likely to contain gems than chemical sedimentary rocks.
2) Chemical sedimentary rocks might make excellent display samples showing crystals, etc., but why are they not generally good for cutting gems? What properties might be missing?
3) A golden looking sample is found in a stream. What test might you run on it to see if it is valuable?
4) Illustrate and explain where a placer forms in a stream.
5) Would a grain of gold generally be the same size as a grain of quartz found in an alluvial deposit? Look at the densities of both and try and explain their hydraulic equivalency (water moving equivalency).
6) Gold is now being found in “noseeum” deposits. But in the past it was found mainly in stream deposits as nuggets. What is a nugget? What is the biggest nugget found?
7) Rounding and sphericity of particles may indicate how far a sample has moved. Explain why the texture of breccia has not moved far, but that the texture of well sorted, rounded sand has.
Illustrate both.
8) In general what are the chemical sedimentary rocks (this includes biochemically formed rocks) composed of? So there might be two types. From your experience of the rocks above from lab, do any of them appear to behave as if they had a detrital origin (have particles)?
9) Explain or give your thoughts on how a detrital rock, such as sandstone, conglomerate, or breccia) derived from an older sedimentary rock would look compared to a detrital rock derived from a granite. You may discuss the mineral contents, etc. This is not an easy question to fully answer. But at least try to explain textural differences. If you can think about compositional differences too.

Review Questions and Terminology


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