I think I understand this correctly, but it’s shocking how many times I have thought that and been wrong!
Ursula puts Elli Horowitz into play, the reaction is a search of the top 9 cards of her deck for a Relic Asset. Moonstone is the Relic found in that search, which causes it to be attached to Elli.
Moonstone says it can’t enter play from your hand, only as a reaction to being discarded.
But in this instance, nothing has been “put into play” because of Elli’s card text. Moonstone is attached to her as a result of the search and is “considered to be in play and under your control”, so any text on Moonstone regarding how it enters play, doesn’t apply right?

So, you're right but sort of for the wrong reasons.

Elli's text definitely causes the attached card to enter play - we know this because the Seal keyword still resolves (if, for instance, she targets the Cthonian Stone; see the developer response here). However, the relevant text on Moonstone is "You cannot play or commit Moonstone from your hand." - Dr Elli Horowitz does not cause the card to be "played" (which is distinct from "put into play" or "enter play"), and even if she did, it's not being played from your hand but directly from your deck, so the restriction doesn't apply. So Dr Elli can definitely get Moonstone for you, and it's one of the better options for her.

6 hours ago, Allonym said:

So, you're right but sort of for the wrong reasons.

Elli's text definitely causes the attached card to enter play - we know this because the Seal keyword still resolves (if, for instance, she targets the Cthonian Stone; see the developer response here). However, the relevant text on Moonstone is "You cannot play or commit Moonstone from your hand." - Dr Elli Horowitz does not cause the card to be "played" (which is distinct from "put into play" or "enter play"), and even if she did, it's not being played from your hand but directly from your deck, so the restriction doesn't apply. So Dr Elli can definitely get Moonstone for you, and it's one of the better options for her.

Thanks so much . I had it in my head when I was writing that it would be played from my deck and not my hand, thereby making the restriction moot. Not that I did a good job of expressing that! But I am grateful for the added clarification of your response. I haven't played Ursula since the advent of Moonstone, but put it in her deck thinking, “well, I will just have to hope I get the cards that give me extra cards”, when suddenly it dawned on me. Elli is the way around Moonstone’s restriction, (if the cards align). Plus, with so many relics using accessory slots, she helps there too. I thought I had better check my understanding first though, before launching into a celebratory dance party.