Fourteen items have exactly one way in, and three have a way that exists only for them
Nether Star is the only Boss Drop, Dragon Egg the only One-Time item, Command Block the only unobtainable one. Plus the fourteen things no chest, craft or trade will ever give you.
Most items in Minecraft have several routes in — mine it, craft it, buy it, or find it in a chest. A minority have exactly one, and when that one route is closed to you the item simply is not available. Sorted by how you get them, the sources fall into eleven classes, and three of those classes contain a single item each.
The three classes with one member
Boss Drop — Nether Star. Nothing else in the game is classified this way. It comes from defeating the Wither and drops exactly one. There is no chest that contains it, no villager that sells it, and no recipe that makes it. One Wither, one star, one beacon.
One-Time — Dragon Egg. It appears once, on top of the exit portal, immediately after the first Ender Dragon kill. Respawning the dragon does not produce another. This is the only item in the game with that property, and it is why the egg is treated as a trophy rather than a resource — there is exactly one per world unless you bring in a second world.
Not Obtainable — Command Block. Available from the creative inventory or /give and by no other means. It is worth having a class for, because "you cannot get this in survival" is a different answer from "you have not found it yet", and a search that returned nothing would look like a gap in the data rather than a fact about the game.
Fourteen items are mob-drop-only
No chest, no crafting recipe, no trade, no fishing — kill the mob or go without:
Blaze Rod · Ghast Tear · Totem of Undying · Trident · Prismarine Shard · Prismarine Crystals · Shulker Shell · Wither Skeleton Skull · Turtle Scute · Phantom Membrane · Wind Charge · Breeze Rod · Rabbit's Foot · Spider Eye
Most of that list is gated behind a place as much as a mob: Blaze Rods mean a Nether fortress, Shulker Shells mean an End city, Prismarine means an ocean monument, Breeze Rods and Wind Charges mean a trial chamber. Turning difficulty down to Peaceful therefore closes off most of it in one move, which is the biggest practical consequence of that setting and rarely how it gets described.
Two of the fourteen survive Peaceful, because their mobs are not hostile:
- Turtle Scute is the strongest exception — a baby turtle drops one when it grows up, so no
killing is involved at all.
- Rabbit's Foot is a rare drop from killing a rabbit, which is a passive mob. Still a kill,
but not one Peaceful prevents.
The other twelve genuinely do vanish with the difficulty setting.
The two narrow sources are narrower than they look
Bartering with piglins is often described as a general-purpose gold sink, but for genuinely hard-to-source items it yields three: Ender Pearls, Saddles and Nether Wart. Everything else it gives you is obtainable more reliably somewhere else.
Fishing produces four worth fishing for: Nautilus Shell as treasure, and Saddle, Name Tag and Slimeball out of the junk slot. Saddle appears on both lists, which makes it unusually flexible — bartering, fishing or a chest, but never a crafting table.
Why "one source" is the number that matters
Across the tracked items, 40 of 60 have exactly one source and only three have four. That skew is the useful thing to carry: for most items the interesting question is not which of several methods is fastest, it is whether the single method is open to you at all — the right dimension, the right difficulty, the right structure found.
When an item seems impossible to get, the answer is usually not a better farm. It is that its one route runs through somewhere you have not been.