I have this concept, it's being developed, and the only item I thought of so far is a sword so hence "The Riddle Swords".

Landra's Riddle Sword - Landra, a great Wizard felt slighted by her warrior lover who she thought was cheating on her. When he went out to battle she, out of fidelity, worked with a great Smith to forge a sword and she embued it with a +1 magical enchantment and worked into the sword a riddle.

"What fire hath wrought, shall reveal what's sought."

Legend has it Landra's lover never understood the riddle. And never settled matters with Landra because he never sought her. He is rumored to have died in battle, but the stories all change wherever you go. Most common people tend to think the riddle was some sort of charm spell on the wielder. But it must have worn off because there are no obvious charm effects.

Rules:

1) if the sword is broken, or needs to be reforged in any major way, then the riddle is broken but the swords' +1 properties may be salvaged if repaired.

2) a player may occasionally investigate the sword. Occasionally is at the DMs discretion but it is assumed the investigation is an unusual keen interest in inspecting the sword the way a warrior might when oiling and sharpening. Its not frequent, but it is occasional that the warrior might notice something very unique.

The player must ask the DM to investigate their sword. If the player succeeds a DC 25 investigation they will notice at the tang where the hilt may have worn very slightly that between the tang's metal and the blades metal is another metal very thin.

If the player thinks to test the metal they determine it is lead.

Regardless of the investigation if the player or anyone decides to divine what's inside the sword as if it were a hollow object they would learn they cannot see beyond the layer of lead, though they would have to understand how lead blocks divination for them to understand why.

Furthermore the use of detect magic reveals exactly what one might expect. A magic sword.

3) if the player decides to melt down the sword's outer metal layers and the lead layer will naturally melt with it, then they reveal a rare magical item and have solved the riddle. It will require a cost in materials and gp assuming the melting of the layers of metal need to be precise so as not to damage what they sheath. It is only necessary to be precise with the known materials and is assumed whatever may be inside has a higher melting point, but probably not by much. Hence the great skill and cost required. 

If the sword is considered uncommon then the magical item hidden within it is rare.

That item is probably a dagger with +1 (though I'm not sure it has to be a dagger) and one new ability that I'm also not sure of.

I was thinking the ability could be Luck, giving the bearer +1 luck point like the luck points in the lucky feat that can be used following the same rules. And it applies as long as the item is worn or carried somewhere on the person.

Or perhaps to roleplay, "as the magic of the riddle is revealed, the person who solved the riddle feels like they are a little bit more lucky, as if they were to play a game of dice maybe they will get one more chance to roll or maybe they would force their opponent to roll once more. A blessing to be given to Landra's lover, if he truly sought her."

----- The next item in the series to be edited in -----