Runestone of Ocean Currents

Wondrous item, uncommon
  An ancient, malachite runestone from the depths of the sunken city. This stone thrums with energy and leaves whichever pocket or pouch it is stored in wet with frothy saltwater. This item has 3 charges and is recharged when a spellcaster performs an hour-long ritual, during which it expends a spell slot of 1st level or higher.
  Saltbolt. As an action, you can speak the stone’s command word, expending one charge and causing a frothy, turquoise bolt of force to strike any target within 120 feet. This bolt deals 1d6 force damage and 1d6 cold damage. A creature under the influence of the shield spell, or similar magic, takes no damage from this effect.
  Rare variant: Increase the force and cold damage to 2d6 each.
  Very rare variant: Increase the force and cold damage to 3d6 each.
  Legendary variant. Increase the force and cold to 4d6 each.