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Smell of sulfur and what I can see in the picture tells me you have a rock with arsenopyrite in it. The coloration of the rock indicates a lot of iron is present as well, likely from the decay of the pyrites in the rock. The high iron content is whats setting your detector off. If there is anything else present in it you'd have to crush it and find out or have it assayed. As to worth, only as a yard piece. If you can smell the sulfur its pretty active and not a great idea to keep indoors.
Just crush a sample fine and pan it out. No gold then stop. Gold then take a look in the prospecting forum for ideas on what to do next.