Good question. I'm not sure, but I suspect it shouldn't do anything interesting; mercury allows oxygen to move through the otherwise impermeable aluminium oxide crust and "rust" the metal.
Since sapphire is already 100% aluminium oxide, but arranged in a crystal lattice I think it should be stable in contact with mercury unless the mercury disrupts the lattice (a possibility--my physical chemistry is weak).
Well, sapphire is quite easy to obtain, and I believe that's also true of mercury. I'd be a little hesitant to expose mercury to the air near myself, though. :/
The vapor pressure goes up surprisingly fast with temp, such that a contaminated cold room temp in the winter might be "eh" but it could get bad on a hot sunny summer day. Modern home insulation probably doesn't help much.
Another comment is sulfur is cheap and widely available at garden stores etc and cinnabar toxicity is greatly exaggerated, because historically anyone who mined it, smelted it, and the smelting is a very effective way to get killed just by being nearby. I wouldn't eat it intentionally, etc, but after sulfur neutralization its mostly harmless. Its a much lower vapor pressure and relatively stable compound compared to elemental Hg.
And it'll amalgamate with gold jewelry, if I recall. I'm old enough to have played with Hg bare handed as a kid. Its kind of like gunpowder, respect it and you'll be safe, make a habit in your life of doing dumb things and nothing is ever safe.
http://www.popsci.com/scitech/article/2004-09/amazing-rustin...