Thats one of those old cast buttons made in the image of a dated Spanish Pillar Dollar. Might be from a cuff link. Where did you find it?
http://www.artifacts.org/patriotic.htm
... these dated, facsimile coin cuff link buttons' central devices consist of the symbol of Spain's global empire. Beneath the Spanish crown, two worlds — the old and the new — are flanked by the Pillars of Hercules, the ancient name for the Straits of Gibraltar, the channel that separates the Atlantic Ocean from the Mediterranean Sea through which Spain's merchant and military mariners sailed to reach the Western Hemisphere. These pillars gave rise to the English nickname for Spain's eight real standard of silver currency: the "Pillar Dollar."