The hue of the orbs was the most brilliant of black, and, far over them, hung jetty lashes of great length. The brows, slightly irregular in outline, had the same tint. The "strangeness," however, which I found in the eyes, was of a nature distinct from the formation, or the color, or the brilliancy of the features, and must, after all, be referred to the expression. Ah, word of no meaning! behind whose vast latitude of mere sound we entrench our ignorance of so much of the spiritual. The expression of the eyes of Ligeia! How for long hours have I pondered upon it! How have I, through the whole of a midsummer night, struggled to fathom it! What was it – that something more profound than the well of Democritus – which lay far within the pupils of my beloved? What was it?" (4)
Her "expression" is something like what the French call the je ne sais quoi (the literal translation is "I don't know what," but it's really meant to imply an intangible quality that makes something or someone distinctive or attractive). You know it's there, but you can't find the words to describe it.