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Lecture 15 Notes

by: Hannah James

Lecture 15 Notes Geoscience 331

Hannah James
GPA 4.0
Gems: The Science Behind the Sparkle
Huifang Xu

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About this Document

These are the notes from the last lecture of Geoscience 331 (Lecture 15). They only contain what is going to be on the exam.
Gems: The Science Behind the Sparkle
Huifang Xu
Class Notes
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This 0 page Class Notes was uploaded by Hannah James on Thursday December 10, 2015. The Class Notes belongs to Geoscience 331 at University of Wisconsin - Madison taught by Huifang Xu in Fall 2015. Since its upload, it has received 17 views. For similar materials see Gems: The Science Behind the Sparkle in Geology at University of Wisconsin - Madison.


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Date Created: 12/10/15
Gemstones and You Lecture 15 1 Wisconsin Gemstones 0 Wisconsin doesn39t have a large collection of gemstones the geologic history has been pretty tame 0 But there are some nice tourmaline crystals in some of the igneous rocks in the northeast portion of the state 0 A pegmatite vein with some pin tourmaline crystals occurs nearby Pine Creek No hammers or collecting at this locality 0 Copper minerals are also common quotup northquot I In this part of the state the big mineral story is hydrothermal lead and zinc minerals I These were economically important minerals early in Wisconsin39s history now it39s just too expensive 0 But the state39s motto quotThe Badger Statequot comes not from the feisty mammal but rather from lead miners they would live in holes in the hill rather than spend time and money building a house 0 Here in the lead zinc district which also includes Iowa and Northern Illinois uorite pyrite and other semiprecious minerals are fairly common I There are also some areas that have nice quartz crystals but don39t expect to find rubies emeralds etc 0 There are some nice moonstone alkali feldspar crystals near Wausau but these areas are mostly private property 0 There is a beautiful moonstone on display in Geology Museum 0 The north shore of Lake Superior has many agates these formed in cavities within basalt that poured out 0 Thompsonite discussed earlier is a common gem material from the Keweenaw peninsula in Michigan I This is a soft opaque mineral so it is usually polished into cabochons 0 People find a few diamond grains in the state I Obviously these diamond crystals were brought to the state by glacier 0 Wisconsin gemstones are not terribly common or easy to find 11 What to look for I When purchasing gems you should keep a few details in mind I Use a 10X loupe magnifying glass to examine the stone 0 These small magnifying glasses sometimes called triplets or hand lenses allow you to look at each small facet and see tiny inclusions inside the stone 0 Most deep blue topaz crystals have been irradiated I It is very rare to find this in nature I However since there are fairly stringent controls on how much residual radioactivity is too much these stones are pretty safe I When first looking at a stone check the general shape 0 Look for symmetry Are the table and culet facets centered 0 If it is a brilliant cut is the shape a true circle I These things can have a serious effect on the overall value if the stone is obviously misshaped 0 Look for improperly placed and polished facets I Each facet should be even and meet at specific angles I Each facet should be polished smooth no pits or linear scratches should be visible 0 Check for cracks chips and inclusions I Inside the stone cracks will show up as irregular re ective surfaces sometimes they may have a little arcshape and a rainbow re ection I Chipped surfaces will feel rough when handled but may need a loupe to be seen easily I Inclusions are difficult to avoid O I If they are placed near the edge of the girdle the stone will look very clear 0 Inclusions near the center of the stone will have the largest effect on value 0 Go to the source I There are several national and international organizations that are concerned with full disclosure and the buyingsellingcreation of gemstones I Consult these places for some uptodate information I Treated and synthetic stones and their identification and disclosure is a large concern I A certified gem is one that has been looked at by an accredited gemologist 0 The certificate contains an official 39grade39 such as discussed in lecture 12 and quality report I The certification process takes money but for buying and selling stones the extra assurance that you are buying what you think you are can be worth it o The Gemological Institute of America offers education and certification services


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