Much of the time I think of items as addons to a build, and focus on the race, class, and feat progression when planning. However, in both Westmarches and many standard canpaigns, it is not unreasonable to be able to buy, craft, or at least request some specific items, at least going up to Rare rarity. That got me thinking about trying to optimize a build around the item, rather than the other way around.
Out of the Abyss provides a unique and interesting Rare item, the Stonespeaker Crystal. It has 10 charges, regains ~7 per day, allows spending some charges to cast the three Speak With spells, and, most importantly, allows expending a number of charges equal to a divination spell's level to replace one consumed spell component. The following divination spells have consumed spell compoments:
- Divination (4) 25gp¹
- Fortune's Favor (2) 100gp
- Legend Lore (5) 250gp
- True Seeing (6) 25gp
¹uses two components with combined cost of 25gp, so the crystal can only replace one of them.
If you play with EGtW, the obvious use case for this is the Graviturgy or Chronurgy Wizard, getting Fortune's Favor (upcastable for more targets, letting you safely cover a party at least once per day with a 1-hour long Lucky d20 each). You also get Legend Lore (cast for free once per day to keep improving your results about the same thing) as well as True Seeing as a Wizard, and since TCoE expanded Wizard spells to include Divination, you don't even need Ritual Caster Cleric to get the full set. It's nice to have the crystal on a Wizard who normally doesn't get Speak with Animals or Speak with Plants too.
The most broken use of this item that I can think of is, conveniently, with Chronurgy's Arcane Abeyance on Fortune's Favor. Cast Fortune's Favor at 5th level on four members of the party, and then also save a bead with a 4th level casting to re-up three party members' Lucky dice mid-combat, increasing the likelihood they actually use them. Between that, Chronal Shift, Silvery Barbs, and Convergent Future, you might as well be the DM now.
Even without EGtW, a Divination Wizard can benefit greatly from the Stonespeaker Crystal thanks to Expert Divination, effectively reducing the total burned cost of Legend Lore to one fifth of a spell slot. Again, after enough castings, you at least know as much as the DM does, and with Portent and Silvery Barbs can control the outcome somewhat.
Did I miss any synergies worth exploring?
Note that while Augury has a costly material component, the component isn't consumed.
I think you covered everything that's relevant. It's a neat item in that it strongly incentivizes you to throw out divination spells as much as you can. Great for information gathering, but not a lot of combat relevance, and I'd expect the wizard to use it for a few days/weeks to get a bunch of information about the current plot and then attune to something else as he gets more magic items and more gold that makes just paying the price for new required divinations more reasonable.
Fortune's Favor doesn't seem wildly impactful to me, compared to some of the other 5th level spells you could be throwing around: you could have one reroll for each party member, or you could have a Transmute Rock or Wall of Stone that completely reshapes the encounter.
I think the one place it is pretty nice is on a Divination Wizard where you're not paying the spell slot cost, you're just paying whatever the difference between a 5th and a 4th level slot would be.
I also think that's the only real benefit of Expert Divination here: it's a tough sell casting Legend Lore at the start of a combat day even without the gold cost, it's better to make that the last spell of the day if you didn't need all your slots, or better yet just doing it on a downtime day. Especially since it'd cost you a preparation slot as well.
Optimizing this is probably just "Divination Wizard, cast Fortune's Favor once a day for every combat day, cast free Divination/Legend Lore every downtime day". If you consider that the effect of the item (trade a 5th level for a 4th and the whole party gets one reroll) that looks pretty good if not amazing.