I do not have the book "Gathering Blue" in front of me, but in general terms I can say that art has an element of expression in it that skill does not. For example, if a person is skilled at making baskets, he would be able to make baskets that are sturdy and neat, and which serve their function efficiently. If a basketmaker is artistic, that would indicate that his baskets would also have a quality of uniqueness; they might portray elements of a certain culture, or communicate feelings or aesthetic beauty. Artistry usually goes hand in hand with skill, whereas skill does not necessarily always indicate artistry.
Some might say that skill is something that can be taught, whereas art is an innate talent or a gift given only to certain individuals. Art also is closely associated with creativity, while skill is not, necessarily. In "Gathering Blue", Kira learns the skill of weaving - how to manipulate the tiny threads carefully and neatly - from her mother. She has a gift, however, which is recognized by her society, to imagine unique patterns of her own, and oftentimes her fingers, directed by inner inspiration which only Kira can hear, formulate beautiful designs never seen before. Thomas, too, has the same gift in regards to carving. Kira's and Thomas's skills have a higher, artistic dimension, making them artists instead of ordinary craftsmen.