Green’s Dictionary of Slang

treasure n.¹

1. an admirable person.

c.1530		H. Rhodes Bk Nurture in Babees Bk (1868) 83: A seruaunt to suffer in anger, to his mayster is a treasure; A seruaunt not reformable, [...] Ofte tymes falleth to pouertye.
1860		E. Eden Semi-Attached Couple (1979) 211: What treasures you all are!
1874		H. Mayhew London Characters 313: You have a treasure of a cook.
1901		Boy’s Own Paper 1 June 546: He’s a perfect treasure.
1957		G. Kersh Fowlers End (2001) 66: You’re a treasure, that’s what you are.
1978		F. Norman Dead Butler Caper 73: What an absolute treasure you are.
1983		J. Sullivan ‘Wanted’ Only Fools and Horses [TV script] She’s a little treasure ain’t she?
1997		P. Theroux Kowloon Tong 183: ‘Your house-boy is first class,’ Monty said [...] ‘He’s a treasure,’ Betty said.

2. (also treas, treash) an affectionate term of address.

1920		‘Katherine Mansfield’ Letter 31 Oct. (1977) 194: But, my treasure, my life is ours.
1960		I. Fleming For Your Eyes Only (1962) 168: You know what you just done, treas?
2019		P. Baker Fabulosa 298/2: treash term of endearment.

SE in slang uses

In compounds

Treasure State (n.) [its reserves of precious metals]

(US) Montana.

1934		G.E. Shankle State Names 129: The names Bonanza and Treasure are typical of the mining area of the State, and Stubtoe of the mountainous nature of the western section of the State [DA].
1950		National Geographic Mag. June 693/2: The commonwealth that was lawless wilderness 85 years ago is aptly nicknamed the ‘Treasure State.’ Beneath its mountains still lies untold mineral wealth, though its mines have already disgorged billions of dollars [DA].

In phrases

treasure of love (n.)

the vagina.

1890–1904

Farmer & Henley Sl. and Its Analogues.