We need to differentiate between elements, minerals and rocks - otherwise comparing SG's becomes an apples and pears exercise.
Gold and osmium are both elements - ie they only consist of one chemical entity each. Gold is also regarded as a mineral however - belonging to the 'Native Element' class of most classification systems. I don't think Osmium is found on its own in nature - it is found in platinum ores and as part of the mineral now known as "Iridium" - which has an SG between 19.3 and 21.1, depending upon its composition. Iridium - the mineral - actually consists of several elements combined to a greater or lesser extent but with Iridium - the element - dominant. Either way, Mercury is not the heaviest mineral and I apologise for taking y'all on that voyage up the garden path.
Evenkite would float on water (because its SG is less than water's 1.0) - but I believe it dissolves readily, so this will have to remain as a thought experiment.
Pumice does float on water but, as you surmised, this is due to its physical structure. Furthermore, it is a rock type - rocks being made up of two or more minerals - though technically one could call a lump of talc (frinstance) "talc rock". Minerals are made up of one or more elements.
stales
