A fractal knife is a bladed weapon known for having razor sharp, but fragile, edges.

Ro Laren once obtained a fractal knife from the body of a Cardassian information officer, who had used the knife to torture Maquis prisoners but had been killed by it, after someone stabbed the officer repeatedly with the knife. Despite the fact that fractal knifes were illegal in Federation and Bajoran space, Ro kept the knife when she was assigned to Deep Space 9. Ro brought the knife with her during the mission to Sindorin and indicated that she wanted Taran'atar to be given it in the event of her death. (DS9 - Section 31 novel: Abyss)