A sapphire glacialis is a members-only item that players can use on another player to boost the other player's Defence level. This will temporarily raise the player's Defence level by 4 + 15% of their Defence level (rounded down). They are caught in the Rellekka Hunter area. The right-click option "Release" would release the butterfly from the jar, but this does not affect any player's stats.
Members can make this item by catching a sapphire glacialis using a (magic) butterfly net whilst they have a butterfly jar in their inventory. They need level 25 Hunter to do so, and gain 34 experience points for every sapphire glacialis they catch.
For an exhaustive list of all known sources for this item, see here