Osmium is a platinum group metal and one of the densest known elements. It is used to produce alloys with extreme hardness.
MMC Norilsk Nickel (OTC: NILSY) - The world's largest producer of palladium produces osmium as a by-product.
Anglo American (OTC: AAUKY) - http://www.angloamerican.co.uk/aa/business/platinum/products/ - The South African miner produces osmium along with the other platinum group elements.