ThinkGeek has the Solar Orbit Sterling Silver Necklace on sale for $60 - 50% off with coupon code HALFELF [Exp 12/3 at 2PM PT]+ $9 shipping = $39 shipped. Shipping is free on $35+ so you're better off finding some other cheap stocking stuffer to get to over the $35 minimum.

Includes all the planets made out of stones, plus an "asteroid belt" between Mars and Jupiter. Comes in a gift box. Definitely unique!