MODULE: Physical Properties of Minerals – Instructor’s Guide

Download the Student Handout

CASE STUDY: Diamond or Quartz?

A.      Learning Outcomes: After completing this module, students will be able to:

1. Define the following physical properties of minerals:
   1. Luster
   2. Streak
   3. Hardness
   4. Cleavage
   5. Specific Gravity
   6. Chemical Composition
2. Create and describe tests that can be done for each of these physical properties that might enable someone to distinguish between a specimen of diamond and a specimen of quartz.
3. Given a certain set of circumstances, use reasoning to conclude which test for one of these physical properties is the best test to use in order to distinguish diamond from quartz.
4. Use reasoning to support the conclusion made in #3 above.
5. Use reasoning to support the added conclusion that none of the other tests for physical properties, in #2 above, should be used under the given circumstances of the case.
6. Write a one-to-two-page summary of their conclusions and reasoning used to arrive at these conclusions in a persuasive and understandable manner.

B. Materials: Handout, one per student, of Case Study: “Diamond or Quartz?”

C. Format: This Case Study should be done after the students and instructor have gone over physical properties of minerals and worked with a number of mineral specimens.

1. Students work in groups of 4 – 5, each with a copy of the Case Study.
2. Students discuss with one another the various tests that could be done for the physical properties under consideration.
3. Together, students sort through the various tests discarding those that will either not work, or will violate certain conditions mentioned in the Case.
4. Together, students are to conclude which one test is the best test and why it is to be used to determine which specimen is the diamond.
5. They are also to conclude why all of the other tests are not to be used to determine which specimen is the diamond.       
6. Finally, each student is to submit an individually written one-to-two page report summarizing his/her conclusions and their reasoning for their conclusions.

D.  Assessment: 

1. Assessment will be based on how well the Learning Outcomes were met by the students’ answers to questions on the handout and their individually written report.
2. For grading purposes, the report should be weighted more heavily than the answers to the questions, suggested: 70%/30%.

E.  Answers:

1. The best test: Pick up specimen A and rub it on specimen B. If A leaves a scratch on B, A is the diamond (H=10) and B is the quartz (H=7).  If A fails to scratch B, then B is the diamond and A is the quartz. This is only one test; it is conclusive, and it does not damage the diamond.
2. Specific Gravity: Pick up specimen A in one hand and specimen B in the other hand. The “heavier” specimen is diamond (S.G. = 3.5) and the “lighter” specimen is quartz (S.G.=2.65). The reason for not doing this test is that the rules prohibit doing two tests, and holding both specimens in your hands might be interpreted as two tests.
3. Cleavage: Pick up the hammer and hit one specimen, either A or B. If the specimen that is hit shatters irregularly, it is quartz, which has no cleavage. If the specimen breaks along cleavage planes, it is diamond, which has 4 directions of cleavage. The reason against using this test is there is a 50% chance of hitting the diamond with the hammer and if the diamond breaks, its value will be reduced.
4. Chemical Composition: There are three tests for this property: hydrochloric acid for the presence of carbonate ion (CO3)?, magnetic compass for magnetite, and the porcelain plate for hematite. The reason against using these tests is that neither diamond nor quartz contains carbonate, is magnetic, or contains hematite.
5. Other tests for hardness include using the copper penny (H=3.5), the steel nail (H=4.5), the glass plate (H=5.5), or the porcelain plate (H=6.5). Since both diamond (H=10) and quartz (H=7) are harder than any of these objects, none of these tests will work.