“Aloft” for both Bilbo’s tiny light by which he finds the Arkenstone and for holding up the Arkenstone itself.  The “a-” comes from the old phrase ‘on loft” and the “a-” preposition 1 use, position within a situation: the object is in the situation of being up there in loftiness.  Or in the loft, if you prefer.

• 13.018 start across the floor holding his tiny light aloft.
• 17.008 and held aloft the jewel.

“a, prep.1.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.

“aloft, adv. and prep.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.