Amygdaloidal basalt, infilled with chalcedony and chlorite (black infill), Michigan USA. Source: http://www.mindat.org/photo-1137645.html
amyg basalt chalc chl
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A Blog About Stone Gathering, Tumbling and Polishing, and Rocks and Landscapes, from New Zealand – With Musical Interludes (john.tumblestone@gmail.com)