Just as the devil has the best songs, mother nature has the best gadgets. How about a non-toxic, durable and reusable device that retains cold, and can release it to your drink without diluting it or affecting its flavor? Yep, she's got it covered: "Nordic Rock," which does just that.
I can't find reference to "Nordic Rock" anywhere, but this stuff is probably some kind of calcium carbonate (used in aquariums because it won't poison anything). Pour some acid on it to find out! Mocha sells a set of 10 for £16.