Amethyst spikes are the highest level of all Spikes. Spikes are structures that act as walls that inflict damage to all players upon contact, except to whoever placed it and his/her teammates. Spikes will also inflict a small amount of damage on players each time they attack it, even if they are out of range of the actual spikes. When placed, mobs/players can't walk through it, which is very useful when running away from an attacker, or building a base. Amethyst spikes can only be broken by hammers or amethyst tools. Hammers are generally faster at breaking amethyst spikes than amethyst tools. Currently, amethyst spikes are the second-most damaging item in the game, doing 50 damage per second on contact, second only to Reidite Spikes, which do (this is just a guess) 60 damage per second.
Amethyst spikes are crafted with 40 Amethyst, 30 Stone, and an amethyst wall at the Workbench. They are the highest tier of spikes and are one tier above Diamond Spikes. They are used in many highly advanced bases, the only downside being their lower health than doors and walls.