Making this a concentration spell was a terrible choice. A spell designed to absorbed damage that can be completely negated the first time you take damage!!?

Today I learned that most people use support spells exclusively to support themselves.

"This 6th level spell with 1/6th the duration and can only target self has situationally a slightly better effect and doesn't have a gold cost associated with it, so that makes this much lower leveled spell that is available to far more classes/subclasses completely pointless."

"Having concentration on a protective spell is stupid because you need to get hit for it to be useful." No, the recipient of the spell needs to be hit for it to be useful. If it wasn't concentration, you could just fling it on the entire party and have a virtually unbreakable non-magical physical resistance on everyone. If it didn't have a gold cost, you could just spam it, keeping it up on your tank. If you're not the "I cast fireball" type spellcaster, you won't be using this on yourself. If you are the "I cast fireball" type spellcaster, why are you even looking at buff spells besides mage armor? Nobody would play any other class if the druid/wizard could do everything while also being unkillable. That's why Investiture of Stone doesn't have a cost associated beyond the fact that it uses a 6th level slot; if you're worried about concentration you're probably mostly going to use that the supplemental effects... But even then, it wouldn't be too unheard of for the caster to automatically succeed on any concentration check when taking 21 or less damage (which gets boosted to 42 from physical attacks with this or investiture active) by the time investiture is a choice...

I guess healing word is useless because it heals less than cure wounds? Same goes for the mass healing spells since they don't heal you as much? And why would ANYONE take telepathic bond when you can already talk to yourself for free? What does bardic inspiration even do?

@LegendOfZub

Ok, Let's ignore the history of the spell, make no assumptions about its intended use, and respond to some of your points.

"This 6th level spell with 1/6th the duration and can only target self has situationally a slightly better effect and doesn't have a gold cost associated with it, so that makes this much lower leveled spell that is available to far more classes/subclasses completely pointless."

• The investiture spells are generally considered to be quite weak by the community at large. They are both very weak options. However if a player were in the position to cast both spells, Stoneskin would indeed be invalidated. How often do you need resistance for an hour? Access to other classes and sub-classes would only be relevant if the spell was good, but let's pretend it is. Aside from the random cleric subclass that gets it, the only classes that don't have access to Investiture of Stone but do get Stoneskin are half-casters. So at 13th level, two levels beyond where full casters are getting the compared 6th level spells, they get 7th level spells now. At this point it isn't even worth preparing. What could you possibly be fighting that only deals non-magical bludgeoning, piercing, and slashing damage at 13th level? Most games don't even reach this tier of play.

"Having concentration on a protective spell is stupid because you need to get hit for it to be useful." No, the recipient of the spell needs to be hit for it to be useful. If it wasn't concentration, you could just fling it on the entire party and have a virtually unbreakable non-magical physical resistance on everyone. If it didn't have a gold cost, you could just spam it, keeping it up on your tank. 

• Concentration is probably the most significant limiting factor when it comes to spell balance. A spell requiring it invalidates synergy with the other most powerful spells in the game, and tacks on the opportunity cost of your most powerful spell options. One would expect a spell that requires concentration to be either very powerful, or have game breaking synergy. This is not the case, as resistance to nonmagical bludgeoning, piercing, and slashing damage isn't that effective. The majority of enemies are not limited to that specific damage type. Add to that the material cost. While not extreme it's significant. Diamond dust is not available everywhere. This drops the spell from situational, to situational and unreliable. 

If you're not the "I cast fireball" type spellcaster, you won't be using this on yourself. If you are the "I cast fireball" type spellcaster, why are you even looking at buff spells besides mage armor? Nobody would play any other class if the druid/wizard could do everything while also being unkillable.

• That's a pretty big generalization there about how people play, but I can respond to what I think your point is. It's an old and valid one. Definitely, previous editions had issues with casters gradually outshining their counterparts, and spell options were a part of that. However there is a grey area between busted and useless which we call "balanced". As you can see no one is calling for this spell to be removed, it is simply not worth your resources in it's current state.
  
  

(...)if you're worried about concentration you're probably mostly going to use that the supplemental effects... But even then, it wouldn't be too unheard of for the caster to automatically succeed on any concentration check when taking 21 or less damage (which gets boosted to 42 from physical attacks with this or investiture active) by the time investiture is a choice...

• Perhaps, at the cost of their ASI and feat selections they could improve their CON saves by 11th level. That's a concession a player would need to make to have the spell not be trash. How would they be faring in-between 7th and 11th when a DC 10 CON save is very failable? What if they don't spend their ASIs and feats to improve the save, is that DC 10 CON save any easier to make because you're higher level? These are resource costs being spent beyond the spell requirements just to ensure the spell doesn't fail.
  

Some other things you may not have considered:

• Regardless of what you think the spell's intended use is, the fact of the matter is that it's not a good idea to cast it on yourself. Considering that martials have superior defense and survivability, it would need to be a significant boost to be worth expending your resources to improve someone else in an area you're already lacking in.
  
  
• Even at it's earliest availability, you have much more effective choices for your 4th level slots. Not just offensive or utility spells, but specifically support spells.
  
  
• This is tactically unsound. Healing, damage prevention, and buffs are fine if they don't decrease the action economy of players during a fight, otherwise you should ignore them unless a condition is debilitating and affecting the DPR. Considering that enemies and players don't lose effectiveness until they are down, Unconscious is effectively a condition to be removed during a fight. 9 times out of 10 it behooves you to keep your damage going to drop the enemy team before they drop you. This is the reasoning behind the community's high value of healing word. Yes it heals less, but it will up the DPR if you bring a player back into the fight.

Concentration AND a non trivial material cost is a big oof when you still have a fairly decent chance to fail your very first con save after taking any damage. 

Why both? Either or would make this a bit better, but as it stands this spell is just as useless as barkskin lol

This spell might have been worth using if it wasn't concentration.

In fact, I'm houseruling that for my games now. Diamond dust worth 100gp and concentration is way too much of a price to pay to resist non-magical bludgeoning, piercing, and slashing damage.

This spell would be WAY better if it also added +2 to AC.    Otherwise I'd take Greater Invisibilty as it gives disadvantage on enemies attack rolls which is better at  preventing melee damage altogether.

Everyone is talking about how this is awful for being a con spell, but if you use this as a support spell for a party member that's some power right there.

I would remove the expensive material component.  

Spell casting words are:

Mineralus Membranicus

Stoneskin has just become more and more underpowered with every edition after 2nd.  The duration, requisite concentration, material cost, and resistance vs purely non magical physical attacks is basically garbage.  Count on "One D&D" making this even worse.

There's a lot of good points in this comment section. Stoneskin just got added to my list of house rules for spells:
Stoneskin either consumes the material component or requires concentration, not both.

Quote from moralhazzard333 >>

I don’t think it costs concentration.

This spell seems REALLY strong. There aren’t many ways to get resistance for player characters. Even heavy armor mastery only gets you a 3 damage reduction. This spell has to be expensive otherwise it will seriously infringe on a corner feature of the barbarian.

 Literally has the Concentration C next to duration.

If it wasn't for the Concentration requirement I think this spell would be fine.  With concentration it gets left by the wayside.

Personally I think nonmagical BPS being allowed to matter more could help too.

Imagine what a CR 30 that can only deal nonmagical BPS could possibly be.

At 4th level this is more like a CR 7-15 spell.  Not sure where CR 30 is coming from.  That being said there are plenty of monsters that don't use magic weapons that have high CR.

How would ye lot feel about expanding this to resist ALL damafe of ALL kinds.

Great spell

This spell is great, it's just situational.  

Just multiclass into barbarian. One hour of resistance is only marginally more valuable than multiple minute-long uses for a combat ability. Yes it's a little more limited since you also can't cast other spells but it's still way better than having to pay the 100gp tax every time.

I honestly don't see the point of this over level 1 shield, ok it only lasts for a round, but nearly all forms of BPS damage require attack roles, and buffing AC by 5 is going to be increasing it to like 24 or 25 by level 7 when this spell is available, since anyone taking this spell is going to be making a spellcasting tank and will also want AC anyway. At higher levels shield may become less useful due to the increasing attack roll bonuses, but it is pretty much always good, especially against huge hoards where each individual enemy has a low attack roll bonus. The only thing I can think of for this spell would be synergising it with armour of agathys to get more uses out of the temp up, but even then it would be better to get 1 level of multiclass into barbarian.