Do chests appear randomly, or are they rewarded after you achieve specific things? What achievements? The odd distribution (I've gotten 8 Fire Breather masks, and only one of most of them) hints that it isn't random.
The ten rings are Infinite Loop, Angel Wings, Twisted Root, Floating Island, Sky Clasp, Demon Claw, Ruby Shard, Caves Maw, Mystic Rocks, and Idol's Curse.
The ten masks are Fire Breather, Infinite Seeker, Traveling Tiki, Skyward Scream, Serpent's Smile, Silent Menace, Jade Monster, The Gatekeeper, Demon Skull, and Face of the Idol
The five Christmas artifacts are Candy Cane, Stocking, Holly, Present, and Wreath.
Also, have you ever opened more than one chest per run? I ask because all this work to reach Level 10 is making me kill myself after I grab a chest.