apple (of the eye), black, obscure

Boyd’s Bible Dictionary:

(bursting forth, in Hebrew). The fruit is alluded to (Prov. 25:11; Song of Sol. 2:5; 7:8). Tree mentioned (Song of Sol. 2:3; 8:5; Joel 1:12). For figurative use see Prov. 7:2; Zech. 2:8; Psa. 17:8; Lam. 2:18.

Concise Bible Dictionary:

This is generally supposed to refer to the citron but apples grow in Palestine, and the Arabic name for the apple (tuffuh) differs little from the Hebrew word, tappuach. Others believe the quince is alluded to, which is fragrant and of a golden color. (Song of Sol. 2:3,5; Song of Sol. 7:8; Song of Sol. 8:5: Joel 1:12). In Prov. 25:11 “a word fitly spoken” is like some elegant device, as “apples of gold in pictures [or baskets] of silver.”

Strong’s Dictionary of Hebrew Words:

Transliteration:
’iyshown
Phonic:
ee-shone’
Meaning:
diminutive from 376; the little man of the eye; the pupil or ball; hence, the middle (of night)
KJV Usage:
apple (of the eye), black, obscure