Percy describes them as "wicked" long hunting knives. Like diamond bladed swords, the knives of Artemis are bewitching and deadly. They are well known for their charms as much as their mercilessness. The material that the knives are made from is silver, which is a color and symbol of Artemis and her hunters.
The knives are the primary weapons of Artemis when fighting in close combat and are presumably also used during her hunts.