Well, there's still the issue that gallium is liquid at 30°C. So if you cooled it somehow and then a person wore it as a pendant it would start dripping. Could be a gallium alloy? I have no idea about any of those. The mercury amalgam idea seems implausible to me, because the densities would probably be too high for the mass of that ball. Unless that number is way off. Still, even a mercury-copper amalgam wouldn't be all that cheap...