Stone of Jordan is a Unique Ring in Diablo IV Season 15 for All classes. It comes from Season 15 Legacy Unique; Horadric Cube: Item Transmutation - Upgrade to Unique from a Common base of the same type; All Unique items can drop at level 1 in any Difficulty since 3.2.1 (official patch notes).
Unique power
Your Resistances are each equal to your highest, and you deal [15 - 25]%[x] more damage of that Element.
Affixes
+Primary Core Stat ([83 - 99])
x% Cold Damage Multiplier (x[7 - 10]%)
% Cooldown Reduction ([5.0 - 8.0]%)
+ to All Skills (guaranteed affix) ([1 - 2])
Where it drops
Season 15 Legacy Unique
Horadric Cube: Item Transmutation - Upgrade to Unique from a Common base of the same type
All Unique items can drop at level 1 in any Difficulty since 3.2.1 (official patch notes)
Legacy Unique: general Unique pool, Uber bosses, Horadric Cube Mythic crafting (permanent)
General Unique loot pool / world drop - not attached to any boss loot table (one guide, another guide and a third guide)
Nightmare Dungeons, Helltide Chests, World Bosses, Kurast Undercity, Purveyor of Curiosities (Murmuring Obols) (one guide)
Uber/Lair boss ladders and Tormented bosses, Helltide chests, Whisper caches (one guide and another guide - low authority, unverified)
random drop - not listed on any Lair Boss loot table in the community loot-table cheat sheet this dataset uses; no official Blizzard list confirms it