Well, since Kryptonite is an imaginary substance, and imagination is weightless, that probably won't do much good.
Krypton is a noble gas, and air doesn't weigh enough to make any difference either. Krypton difluoride, sometimes referred to as Kryptonite, is a volatile, colourless solid; with a density of 3.24 g/cm^3. Volatile compounds probably aren't the safest things to use as ballast.
Wikipedia says that, "a form of the fictional mineral "Kryptonite" has been discovered recently in Serbia; it has been formally named Jadarite (after the region where it was discovered), molecular weight 221.63 gm. The composition of the mineral is sodium lithium boron silicate hydroxide, which is one element short of the description of Kryptonite in Superman Returns (the mineral lacks fluoride)" so I guess you could call that pseudo-Kryptonite. Anyway, it's kind of hard to obtain.
Also in Wikipedia is the Superman III (1983) formula for Kryptonite that is "a compound made up of 15.08% plutonium, 18.06% tantalum, 27.71% xenon, 24.02% promethium, 10.62% dialium, 3.94% mercury, and 0.57% of an unknown substance." However, this is obviously fictional as while Xenon, a noble gas, is known to form many compounds, is not capable of forming a compound with these particular elements.
