Argon Coin: The Golden Fleece (金羊の皮, Kin'yō no Kawa?) is a coat of Golden Fleece from the winged ram Chrysomallos, given to King Aeëtes of Colchis by Phrixus as a token of his gratitude. It was possessed by Medea, the princess of Colchis, and she was made to blindly love Jason, who was sent to Colchis to retrieve it, by the goddess Aphrodite in order to defeat King Aeëtes. She nullified her father's magic, allowing Jason to obtain it. It is inhabited by a strange Guardian Spirit, similarly to the Seventh Holy Scripture. Caster does not possess it as an active or sealed Noble Phantasm, but instead as a Skill.
It is very valuable as the hidden treasure of Colchis, and while there is nothing special about the fleece itself, there is an anecdote that a dragon will appear if it is thrown on the ground. The exact reason isn't known, but it seems people have mixed up the Golden Fleece with the dragon that used to guard it, making the story feel like some kind of satire. While it is capable of summoning the original Colchis dragon that guarded it, Caster does not have the knowledge or skill to summon a Phantasmal Beast, so it goes unused during the Fifth Holy Grail War. If she did possess the proper skills necessary to summon the dragon, it would bring her up from being the weakest Servant to the middle ranking among the Servants of the fifth war. However, the dragon itself isn't strong enough to be able to make a clear difference against higher level opponents.
The spirit's gender is unset, and was originally supposed to appear in Fate/stay night. Gilgamesh was placed into the story instead, and any involvement with Argon Coin was forgotten. It was only referenced in Fate/side material while they were searching for material to put in its encyclopedia. It was going to be placed into Fate/hollow ataraxia, but it ended up not coming to fruition. It was given some rough art designs in Fate/complete material V where it is shown having a bad relationship with Caster.
 Fate/side material - Encyclopedia: Golden Fleece [Noble Phantasm], p.059 [T]
Golden Fleece [Noble Phantasm]
As the hidden treasure of Colchis, it's very valuable.
Though there's nothing special about the fleece itself, for some reason there's an anecdote that a dragon will appear if you throw it on the ground.
It seems that at some point, people mixed up the Golden Fleece with the dragon that used to guard it.
The story feels a bit like some kind of satire.
Since Caster doesn't have the ability to summon dragons, she can't summon the dragon.
Also, there's a strange spirit living in it, just like a certain Seventh Scripture.
 Fate/complete material III: World material - FAQ with Nasu: Noble Phantasms, p.134
Q: If Caster had a skill (dragon summoning skill) that let her use the Golden Fleece, would she be able to escape the rank of being the weakest Servant?
Q: If Caster had a skill (dragon summoning skill) that let her use the Golden Fleece, would she be able to escape the rank of being the weakest Servant? Also, will the spirit in the Golden Fleece ever see the light of day? He's cute so I wanted to see him in color.
A: She'd move up to the middle rank, I think. Oh, but, the Colchis dragon isn't that strong right....
 Fate/complete material V: Hollow Material - Last talk: Kinoko Nasu X Takashi Takeuchi conversation, p.342
Were there any characters you were forced to omit for plot reasons?
Takeuchi: I wondered if I'd be able to get Argon Coin (refer to p. 217) in somehow, but in the end I wasn't. We wanted to get everything we could find for the content for 『side material』, and by the end of it we were scraping for it.
Nasu: Thinking about Argon Coin now, he/she was in 『stay night』's 『side material』, but Gil was put in his/her place. After that, we lost track of things with Argon Coin.