Precious

Precious is a word with a long history in English before Tolkien made it his own (or Gollum's). It is most widely used as an adjective: the 'OED's entry for the word traces it back to Middle English. But it is as a noun that it has become most strongly associated with Tolkien, and with the Ring: Gollum's use of the word immediately precedes the first appearance of the Ring in The Hobbit, and 'his last wail Precious' marks its destruction in the Cracks of Doom. (Gollum also uses 'precious' to refer to himself, but the Ring is distinguished as 'the Precious', with a capital letter.)

In fact its use as a noun, in the sense that Gollum uses it, also goes back a long way. As a term of endearment, similar to dear or darling, it is first recorded in the Elizabethan tragedy Antonio and Mellida by John Marston (c. 1575-1634): 'Nay, pretious, If youle be peeuish, by this light, He sweare Thou rail'dst vpon thy love.' Not that Tolkien would have known this from the OED entry as he saw it: the First Edition of the Dictionary gave as its first example a quotation from Susanna Centlivre's comedy The Basset-Table of 1706 ('With all my Heart, my Jewel, my Precious'). The example from Marston, only recently added to the OED database, extends the known history of this sense by over a century.

Tolkien also uses the adjective in its familiar sense 'valuable', and occasionally in an ironical sense, also recorded in the OED, when referring in a belittling or depreciative manner to things considered of little or no value — for example, when one of Saruman's ruffians asks Merry where 'those precious Shirriffs' have got to (LotR vi. viii).

The Ring of Words ~
Tolkien and the Oxford English Dictionary
Oxford University Press (2006)