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Bob Harman
 #1 
For Midwest mineral and geode collectors finding a nice example of MILLERITE is one of the "holy grails" of Midwest mineral collecting.    Millerite is nickel sulfide (NiS). It is silvery in color.   Its oxidation products are less well classified and consist of honnessite, jamborite, pecorite and maybe others. The oxidation products are found along side the basic millerite mineral.   Honnessite is usually bright grassy green in color as (to some extent) are the other oxidation products.

Millerite can be found as free standing acicular sprays of just a few strands to larger free standing impressive sprays or it can be found imbedded within calcite (or rarely barite). Or in the so-called "cobweb" variant which is usually a loosely tangled conglomeration of strands in no particular shape.
Gently etching solid calcite filled geodes (use household vinegar or very dilute muriatic acid to limit fizzing) occasionally frees up collector quality sprays of the millerite or its products.

Millerite is found in the Wallace Quarry, Bayport Michigan, At the Harrodsburg road cuts in Monroe County Indiana, the (now extinct) Hall's Gap road cut site in Lincoln County Kentucky, the (also now extinct) Estabrook Park site near Milwaukee Wisconsin, several sites in or near the Keokuk geode area in Missouri and other non-midwestern sites in the East (such as Antwerp, NY).

The silvery acicular spray of "millerite" is virtually always a correct identification by appearance alone, but the bright grass green acicular spray of "honnessite" may not be such a "slam dunk" identification from appearance alone.   Several mineralogist acquaintances have suggested the sprays might be a mixture of the basic millerite and its oxidation products. They advise analysis of a number of the strands. Unfortunately the analysis destroys the strands and overall appearance of the spray so, practically speaking, analysis of collector examples is rarely, if ever, done.  Each strand in each spray might just be a bit different.  
 So silvery spray = MILLERITE. Grass green spray = probably honnessite or ????   BOB
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