Amethyst bolt tips are attached to the tips of broad bolts to create amethyst broad bolts, rewarding 106 Fletching experience per 10 completed bolts. Fletching amethyst broad bolts requires level 76 in Fletching and having unlocked "Broader fletching", costing 300 Slayer reward points.
Amethyst bolt tips are created through the Crafting skill, requiring a Crafting level of 83. Curiously, amethyst bolt tips are the only bolt tips to give Crafting experience instead of Fletching experience when created. Using a chisel on an amethyst and selecting the "make bolt tips" option will result in 15 amethyst bolt tips and 60 experience per gem.