Helen, thy beauty is to me
Like those Nicean barks of yore
That gently, o'er a perfum'd sea,
The weary, way-worn wanderer bore
To his own native shore.
On desperate seas long wont to roam,
Thy hyacinth hair, thy classic face
Thy Naiad airs have brought me home
To the glory that was Greece
And the grandeur that was Rome
Lo! in yon brilliant window-niche
How statue-like I see thee stand!
The agate lamp within thy hand
Ah! Psyche from the regions which
Are Holy Land!
Again tying to the theme of ancient Greece, agate is a valuable type of quartz used by ancient Greeks. They are a symbol of Gaia, the Greek Earth Mother; and Nyx, the night goddess. 
 Agates also symbolize healing, courage, and strength; tying in to the  worn adventurer metaphor  from the previous stanza. 

To Helen