Ring of Arcane Focus (5e Equipment)

Ring, very rare (major) (requires attunement by a spellcaster)

This silver ring has bands of black metal wrapping around it, and a vibrant purple gemstone in the middle. While wearing this ring, you have advantage on saving throws to maintain concentration on a spell. In addition, when you cast a spell that has a duration other than instantaneous, you may triple the duration of that spell.