All stones auto save holder on saves of there ability
Diamond Con
The holder can revive any one creature to full hp per day
Ruby Str
Holder crits on 18-20 regardless of AC
Sapphire Dex
Every initiative the holder chooses 1/2 others and they are first in the initiative order
Emerald Int
+10 to Intel checks
Amethyst Wis
Adds a d6 to party on Wisdom saves, +5 to perception checks
Topaz Cha
+ 5 to Cha, add Cha mod to spellcasting mod anytime it is used.

I want them to be strong but not gamebreaking and as a dm I can plan accordingly. The topaz seems a little weak in comparison. Any ideas?

Also looking for ideas of events to occur once all 6 are brought to there alter. End game stuff though.

Make sure that there isn’t a power gamer in your group to hoard these, or you may have an entire campaign go wrong.

I’d recommend some form of price for being able to even wield a gemstone like it takes immense power in order to wield one. Also maybe the effects you said should be nerfed a bit but the gems give some form of special action like a power. (Maybe this would work I don’t know your world lore.)

Also when they are brought together, then it is a little bit of a stereotype to have something only bad happen when the stones are put together. Maybe instead they are imbued each with a small bit of power from one another and the stones gain new powers (however the power would need to be earned.) Once the stones are together maybe the wielders are put into a very difficult life or death trial to see if they are worthy of even greater power which they may need to defeat the BBEG who could be separate from the trial or the final challenge of the trial (like the bad guy may be imprisoned in the trial but slowly escaping).

It’s your world so I don’t know much about it so it is a little hard to suggest ideas. Overall the gemstones do seem broken for a low level party. I am guessing you are going for a high level party.

Yes presumably they wouldnt be able to obtain 1 until 7 or 8 and each one is going to be rather far from the others. Great ideas though. The trial to be deemed worthy. I like it. How nerfed? I want them stronger than a normal magical item. Essentially infinity stones for d&d. I also like the tax for the use. Will implement that.

I agree. I think any 1 character will only be able to wield 3 at most.

Make them Attunement

Quote from Scion_the_Heir >>

Yes presumably they wouldnt be able to obtain 1 until 7 or 8 and each one is going to be rather far from the others. Great ideas though. The trial to be deemed worthy. I like it. How nerfed? I want them stronger than a normal magical item. Essentially infinity stones for d&d. I also like the tax for the use. Will implement that.

 I’d say nerf the bonuses for all of them but give a one (or a small amount of uses) time per rest ability to be used. It’s your campaign so it’s your choice on the abilities with the lore as well as the powers for each stone. Yeah levels 7-8 is good. Not late in the campaign if going up to 20 but would most likely make sense canonically

If you want something like infinity stones then like think of what each stone represented and what could be done with them. For example the space stone allowed for manipulation of space and matter and the ability to warp space to make a portal.

These are all Legendary and they should all require Attunements.

Non-god like version would look like this:

Diamond:   Advantage on Con save,  the holder can revive any one creature to full hp per day

Ruby: Advantage on Str save, holder crits on 19-20.

Sapphire:  Advantage on Dex save, Holder gets a +10 bonus to initiative.

Emerald: Advantage on Int. save, +10 to Knowledge checks

Amethyst: Advantage on Wis save, Adds +5 to perception checks for all allies within 30 ft.

Topaz:  Advantage on Cha saves, For all Charisma based spells and attacks, adds +3 to any Spell DC AND +3 to any to hit roll

A question: How many players are in your party?

Also I’d recommend giving all party members one stone when players first get stones for PC balance which means you may want to add more stones depending on player count. The amount of stones should be divisible by player number and each stone should be geared for different party members.

If you want the stones to be obtained 1 by 1 then maybe each player could only use a stone for so long before bad stuff happens.

I should have 5 players. I wont allow more than 6.

Here's what I have now.

The PC must have an ability score of 15 or higher in the stones ability to attune to it.
All stones give adv to holder on saves of that ability.
Stones must be brought to their altar to engage in trial to unlock full potential.
Each trial will need the whole party to complete.

Gems to find-
-Diamond Con
The holder can use their bonus action to stabilize up to 2 creatures within 60ft per long rest
Max- The holder can use their action to revive any one creature to full hp within 60ft per long rest, resistant to poison damage and cannot be poisoned.
-Ruby Str
As many times as your Str mod the holder can add a second Str mod to attack rolls after the roll per long rest
Max- Holder crits on nat 19-20, cannot be grappled while conscious.
-Sapphire Dex
Holder adds d10 to any initiative roll once per initiative
Max- The holder is always first in the initiative order and as many times as their dex mod can give disadvantage on attacks against them per long rest.
-Emerald Int
Holder has adv on your choice of Intel check. Can be changed per long rest.
Max- +d10 to Intel checks, can communicate in any language.
-Amethyst Wis
Holder is proficient in all Wis checks.
Max- +2 spell mod. Party adds d6 on Wisdom saves, You can't get lost.
-Topaz Cha
Add Cha mod to any roll as many times as your Cha mod/2 per long rest
Max- +4 Cha, prof in all Cha checks and double if already prof, you are more attractive giving you adv on your choice of Char check per long rest.choice Can be changed per long rest.

The event that occurs once they're all brought together at the altar will have something to do with unlocking a door to something. Maybe planar travel, or teleportation to and from each stones altar, or any other cool utility ideas you guys have.

i've been tinkering with gems a lot lately...tI'd look at volo's guide to all things magical for inspiration (or here)...there's ~20 pages of gems in there.  my own homebrew breaks gems down into 2 categories:

1. gems that provide a protective aura (in which case you need several embedded into your armor or clothing) and
2. single gems that do something (like one on a ring that glows when an invisible creature approaches - or a gem that, when embedded in a sword hilt, gives it a +1 bonus).

I also have a rule that says that for some not totally understood reason, the gem effects within a category don't stack...when in close proximity, one always suppresses the effects of all the others (but still keeping the two categories separate so you can have gems in your armor and a gem in your weapon)....so effectively, you can have one effect from each category. 

The more powerful gems are required to be put in armor/weapons and the gem increases the item's rarity rating by some amount to keep things balanced (and that increase determines the cost of the gems needed so they're not getting a vary rare sword for the cost of a topaz). ....although requiring attunement to the gem would make this unnecessary.

for gems that don't need to be embedded in armor or weapons (like those that can adorn clothing or on a necklace or just sitting on a desk)...the effect is very weak, something equivalent to a 0.5 tier increase in a weapon.

something like a bloodstone would be a more fitting name than diamond, considering the effects....imo anyway