Michigan's State Gem Stone

Chlorastrolite

(Green Star Stone)

Commonly called Greenstone. All it's names come from it's appearance. It varies from a light green, to a green so dark it is almost black, and due to it's property known as "chatoyance" (cats eye) it changes from one to the other as it is turned in the light. The markings on a Greenstone form a pattern described as "Turtleback" and in some ways the pattern is similar to the pattern on a Petoskey stone.

Our Gem stone is a true Michigander. It is found nowhere other than Michigan.

It is sometimes called Isle Royal Greenstone because it was first found on the beaches of Isle Royal. 
Presently, Greenstones are found in the poor rock (waste rock) piles from the copper mines of the Keweenaw Penninsula.
They are usually small, one inch would be large, 1/2 inch and smaller is more typical, and they occur as fillings of cracks and bubbles in the mine rock. 

It is very difficult to find gem quality Greenstones, many show no pattern or have defects which make it impossible to polish them. In addition the pattern in a good stone may be very thin making it easy to polish right through the pattern and ruin the stone.

Chemicaly Chlorastrolite is a variety of the mineral Pumpellyte

It's only use is in jewelery and small decorative items.

 

 

Links for more information:

http://stoneplus.cst.cmich.edu/chlorastrolite.htm

http://www.michigan.gov/documents/hal_mhc_mhm_greenstone_63847_7.pdf