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...The Lone Hill Thundereggs occurred in a rhyolite dome that intruded marine sediments in what is now Los Gatos, CA. Because of the marcasite content in the sediments, these eggs picked up a lot of disseminated marcasite into their makeup, which accounts for the grayish matrix and agate that ranges from almost clear to amber colored. I have even seen some with chunks of what look like marcasite in the agate core. When dry sanded, these eggs smell like sulfur. The site where these occurred used to be a quarry, but has since been filled in and become a housing tract. The largest I have seen is small cantaloupe size... “

written by Mark Ketsdever, Hillsboro, OR