She is glad, now, that her grandmother once taught her to mine and hand-cut gems from the valley's veins of crystal, though she breaks nails and grazes knuckles; for she would have this pledge of faith be the labour of her hands alone.
As she works each gem into its place in the ring of seven stars, she weaves the Grey Company a blessing with it: for Elladan, that his healer's hands stay steady. For Elrohir, that his blade strike ever true. For Halbarad, loyalest friend. For Estel… She cannot find the words. O Lady of Starlight, watch over them!
I can't claim credit for the idea that Arwen mined the gems herself; that, and much of my thinking about Arwen's making of the banner, goes back to forodwaith's fabulous story Thread of Fate, which is here at FFnet and here at HASA.