So, Adamantine is has an AC of 23, so, would you do 23 + whatever armor you have underneath?

This is armor that is reinforced with adamantine, so the player's AC is dependent on what kind of armor is being reinforced (the AC column of the table).

This isn't armor primarily made out of adamantine so it does not include adamantine's 23 armor class.

The way i understan it is that the metal is not inherently enchanted, no plus to ac. A sold block would have a 23 ac. But it has to be crafted and shaped in a way that allows movment. However because of how hard it is even a solid hit cant punch through the plates of metal that make up this armer.

It does NOT give a bonus to AC at all unless it has a +1,2,3 modifier on it. 

 While you're wearing it, any critical hit against you becomes a normal hit.

It only provides the effect of turning critical hits into normal hits 

Adamantine does not seem to be inherently a magical metal, just uncommon. However, there does not seem to be a specific value attached to the armor for that rarity. When it comes to value for sale, the DMG (pg 135) puts Uncommon items at 101-500 gp.  The question seems to be, would you add that number to the value of the armor, or would that number replace it?

You would have to add that value to the normal value of the armor, otherwise if you replace the value number Adamantine plate would cost ~500 gp while regular plate would cost 1500 gp (PH pg 145).  It makes no logical sense for the specialty armor to cost less than the regular version.

There is no addition to armor class here. Adamantine Plate has the same AC value as plate (18) Adamantine splint has the same value as splint (17) etc. You just have the additional benefit of negating critical hits.

I've always taken the approach of adding the cost of the Adamantine onto the armor, but I would normally do a flat 500gp extra. That would make Adamantine Plate 2,000gp. I rarely give it out to players through just because some of them get a little overzealous about their AC being as high as possible when they find out what it can mean.

no its the normal armor ac . adamantine armor only stops crits.

best armor ever

What would youu sell this stuff in a shop for. Like what is conventional?

I gave a hero in my party this armor and I swear on my life that I have rolled twice the number of twenties on him since he has this armor, but of course, since he has this armor,  the critics doesn’t matter. Also a lot of wikis have charts for a good price for a magic item and I’m pretty sure that the DM’s guide has it too.

I think when they say 23 they are refering to the AC of the raw material rather than the armor which it makes sense that the AC goes down when its turned into armor because armor has weak points or leaves uncovered areas so having less AC makes sense

@DavidNovacat You would just add 500 Gp to the value of the armor and then modify the price based on persuasion rolls and the fact that the merchant may try to swindle the party, or at least that's how I'd do it. You can also check the downtime activities for selling magic items in the PHB or Xanathar's if you want to try those. Also, it says in Xanathar's Guide that you can get weapons made from adamantine and it costs the same. For anyone who doesn't have the book, adamantine weapons make it so that attacks against objects are auto-crits and they bypass resistance against nonmagical weapons for a specific few creatures, such as gargoyles and the stronger golems.

question. can armor have multiple magic effects? E.G. admantine armor of invulnerability?

+mithral?

Quote from i_pull_levers >>

+mithral?

 Most likely only one metal. But I believe yes armor can have multiple effects.

Quote from FarwinMageBlood >>

Quote from i_pull_levers >>

+mithral?

 Most likely only one metal. But I believe yes armor can have multiple effects.

 thanks

Does the adamantine have to be used during the forming process or can it be an add on to already formed plate?

If a crit becomes a normal hit does that mean it can fail at your AC, or is it still an auto-hit just with normal damage?