Looks
the face that reunited kingdoms
"In the middle of the table, against the woven cloths upon the wall, there was a chair under a canopy, and there sat a lady fair to look upon, and so like was she in form of womanhood to Elrond that Frodo guessed that she was one of his close kindred. Young she was and yet not so. The braids of her dark hair were touched by no frost; her white arms and clear face were flawless and smooth, and the light of stars was in her bright eyes, grey as a cloudless night; yet queenly she looked, and thought and knowledge were in her glance, as of one who has known many things that the years bring. Above her brow her head was covered with a cap of silver lace netted with small gems, glittering white; but her soft grey raiment had no ornament save a girdle of leaves wrought in silver."
-- Lord of the Rings Book Two, Chapter One: Many Meetings
When one first looks upon Arwen, the first thing one will notice is her beauty, which is breathtaking, almost ethereal. Her hair is as dark as a river at night, and her eyes are grey, like the twilight, far-seeing and piercing. Her skin is alabaster white, and her features so finely formed, she is almost like an angel -- unsurprising, as she is actually descended from Middle-earth's closest equivalents of our world's earthbound angels, the maiar. Arwen's looks are meant to be reminiscent to those of Luthien Tinúviel, one of her foremothers, who was the single most beautiful being ever to walk upon Middle-earth. (Luthien is discussed in more depth at Ancestry, Arwen's ethnicity is discussed on this page.)
The artist Angus McBride drew for Iron Crown Enterprises' Middle-Earth Roleplay (MERP) what is arguably the most accurate portrait of Arwen in all Tolkien fandom, shown to the right in its sketch form. Here is what I can pick up from his notes on the side of the sketch --
"Colours alway those of twilight or the evening sky."
"Height, about 5'8". Build; slender and girlish."
Arwen's looks, by Elven standards, are quite rare, considering her ethnicity. Generally it was the Sindarin elves who were dark-haired (Legolas notwithstanding, he and his father are the exceptions to the rule!). While Arwen does have a dash of Sindarin in her blood, her blood is mostly that of the Noldor and Edain (men). She is a glittering, poignant sapphire in a world of diamonds.