Ah, yes... One of the most devastating items in the right hands. And the right hands are:

• Fighter (Preferably Eldritch Knight, but any fighter will do)
• Sorcerer 
• Or anybody multiclassed into Fighter (2 levels for Action Surge) or Sorcerer (3 levels for metamagic, or 5 for fireball)

Why? Because:

Using this item isn't casting a spell. You can't cast 2 spells in one turn, regardless of metamagic or action surge, BUT the necklace doesn't make you cast a spell - it just reproduces a spell (as opposed to, say, Wand of Fireballs, which indeed makes you actually cast a spell).

Therefore:

• Anybody with Action Surge can simply use 2 actions to throw 2 beads, effectively hurling 2 fireballs.
• Anybody with "Quicken Spell" Metamagic and Fireball spell can cast a quickened Fireball, and follow up with tossing one bead.
• Eldritch Knights count as spellcasters for the purposes of attuning to Wand of Fireballs. They can cast fireball through the wand, and then action surge into tossing the bead (tossing only 1 bead a round is better than tossing 2, since they're very much finite, and don't recharge).

And last, but certainly not least, a pinnacle of action economy:

• You can cast 3 fireballs in a single turn if you multiclass and combine Quicken Spell + Action Surge by using a Bonus Action to cast quickened Fireball, and then Action + Action Surge to toss 2 beads from the necklace. I hereby bestow you with this knowledge, and do with it what you will, my lovely min-maxer brethren.

Cast 4 firballs in a turn. first action, throw a fireball, second action (from a haste spell) throw a fireball, third action (action surge) throw a fireball, bonus action (quickened spell) cast fireball.

Quote from hollo_john >>

Cast 4 firballs in a turn. first action, throw a fireball, second action (from a haste spell) throw a fireball, third action (action surge) throw a fireball, bonus action (quickened spell) cast fireball.

 Alas, Haste doesn't provide the option. You can use the action provided by Haste to "Use an Object", but this type of action can't be used to Activate Magical Object (those are two separate action types, precisely for balance purposes).

That's why I haven't included Rogue, Thief archetype in my post. Thieves get "Fast Hands" feature, which allows them to Use an Object as a Bonus Action, but they don't qualify to toss a necklace bead as a Bonus Action.

EDIT: That's why Action Surge is so powerful here. It just gives you an action of any type, without restrictions.

 What would you say a fair trade price for this item would be?

A Pact of the Chain Warlock with Flock of Familiars spell could have up to 3 active familiars who each act on their own.  If the warlock gives each of the familiars to take a bead from the necklace and orders them to attack the BBEG with it, that would mean 4 Fireballs in 1 turn/round.  

Quote from Holvr >>

You can't cast 2 spells in one turn, regardless of metamagic or action surge

You can totally cast two spells in one turn, especially with action surge.

Action-Cast fireball
Action surge action-Cast Fireball

Metal Magic will let you cast fireball as a bonus action and firebolt as an action also but I am going to guess that you weren't considering cantrips as "spells" when you wrote that which fair 

Depends on your party's murderhobo tendencies. More murderhobo = more expensive.

Since it doesn't recharge, you're fair to make it affordable if you just want them to have the option of disposable fireball. What would you charge for a scroll of fireball? Scale it off that.

I don't understand, mechanically, since this isn't spell casting....you're throwing an object like a dagger...if you have multiattack as action like fighters, bards, ect you should be able to throw two per action or if the DM wants to make more challenging...one as an action and one as bonus action.  It's just throwing a bead, not reloading a crossbow or casting a spell.

Quote from Holvr >>

Ah, yes... One of the most devastating items in the right hands. And the right hands are:

• Fighter (Preferably Eldritch Knight, but any fighter will do)
• Sorcerer 
• Or anybody multiclassed into Fighter (2 levels for Action Surge) or Sorcerer (3 levels for metamagic, or 5 for fireball)

Why? Because:

Using this item isn't casting a spell. You can't cast 2 spells in one turn, regardless of metamagic or action surge, BUT the necklace doesn't make you cast a spell - it just reproduces a spell (as opposed to, say, Wand of Fireballs, which indeed makes you actually cast a spell).

Therefore:

• Anybody with Action Surge can simply use 2 actions to throw 2 beads, effectively hurling 2 fireballs.
• Anybody with "Quicken Spell" Metamagic and Fireball spell can cast a quickened Fireball, and follow up with tossing one bead.
• Eldritch Knights count as spellcasters for the purposes of attuning to Wand of Fireballs. They can cast fireball through the wand, and then action surge into tossing the bead (tossing only 1 bead a round is better than tossing 2, since they're very much finite, and don't recharge).

And last, but certainly not least, a pinnacle of action economy:

• You can cast 3 fireballs in a single turn if you multiclass and combine Quicken Spell + Action Surge by using a Bonus Action to cast quickened Fireball, and then Action + Action Surge to toss 2 beads from the necklace. I hereby bestow you with this knowledge, and do with it what you will, my lovely min-maxer brethren.

 I see your agrument with action surge, but like you said, this isn't casting a spell, so sorcerers metamagic or quicken "spell" don't apply.  Anyone hasted should be throw two as the rule stands or three if you use action surge.  I just still argue that if you have multiattack and can throw two daggers per action you should be able to throw two beads.

So basically magic hand grenade. I can see this as useful for any elite military groups based off modern infantry the party encounters... 

Thief rogue can throw a bead as a bonus action and a normal action. Every turn. Until you run out of beads that is. 

Edit: I noticed this was mentioned up top. But I'm curious what makes this using a magic item? You're literally throwing a rock. 

That rock is a magic item... So raw it's using a magic item. Now I would use rule of cool and totally let someone just let lose and throw them.

Edit

Also the item itself states that detaching a bead and throwing it is an action. Tossing multiple beads at once increases the damage not the size or number of targets.

Quote from taven03 >>Also the item itself states that detaching a bead and throwing it is an action. Tossing multiple beads at once increases the damage not the size or number of targets.

 Yep. I’m a dm in a campaign of DoIP into RoT, and tief warlock decided to throw 7 of the beads (rolled a 6 on the number of beads) at the adult white dragon. 9th level fireball obliterated the dragon because it failed the save.

Incendiary grenade necklace! :D 

You can cast more than one spell. You can just not cast a level spell if you use a bonus action spell. You can totally cast 2 fireballs with action surge.

One of my party shot a arrow with a Bead of Fireball attached at a white dragon.  And got a critical hit, I recall.

No one seems to point out that this, by expanding 8 beads or more, is possibly the only way to cast any spell above 9th level.

1d6+3 beads mean there is a maximum level of 9, nothing says beads from multiple necklaces work together... besides, it'd be more cost-effective to do it as individual necklaces as each additional bead is only 1 extra D6 of damage, where the first bead is effectively 8d6 worth of damage.

I've always thought fireball scaled poorly.

Fireball is 3rd level spell, so if you spend 8 or more beads, you will cast it at level higher than 9(3+7 upcast).

Inefficient? yes. But sometimes dealing high damage in short time is higher priority. Also I just find it  interesting that this is one of the two ways to cast anything at 10th or more level.

(The other being a cleric casting a 9th level spell while benefitting from Book of Exalted Deeds, which counts as 10th level)

My Wildfire Druid has this thing re-skined a string of pinecones that incinerate on impact ... a pine-grenade basically. It's a great item.