Pnk Goldeneggs
As of: mc1.12.2-4.9 (Jun-2019)
One of the rarer loot items you will find in a Beanstalk is a Golden Egg or one of MrG's preciouses. You'll find Golden Eggs only in a beanstalk's crown loot box. No one knows how MrG obtained the original golden egg (probably stole it), or how he manages to produce more of them (probably tortures something somewhere). You'll find no geese, golden or otherwise, near beanstalks...just hapless chickens and really mean zombies. But if you are lucky enough to find a golden egg, it will gift you many treasures if used properly. In essense, a Golden Egg gives you a safe way to swipe even more of MrG's hidden preciouses without him even knowing!
The best quality Golden Eggs can be used up to 4 times and aren't bound to a particular beanstalk; these are the ones you'll find in crown loot boxes. An egg's quality translates into the number of times you can use the egg to get at hidden treasure. The number of uses possible for any Golden Egg is shown in its tooltip. A Golden Egg with a single use isn't as shiny as other Golden Eggs and shows a small crack in its shell.
To transform a Golden Egg into treasure simply place the egg like you would a block. Of course, as with all beanstalk-related matters, it's best to place a Golden Egg with as high a Luck attribute as you possibly can. Try wearing some Lucky Pants, wearing golden armor with Jack's Luck enchantment, or drinking a luck potion.
GOOD VIBES! 
less good vibes...
When placed, a Golden Egg usually transforms into one of MrG's hidden prep boxes for the heart loot boxes you find in a normal beanstalk. If your egg can be used again, it inserts itself into the generated loot box in the first slot. Occasionally, your Golden Egg is a dud, and instead of a happy golden box of goodies, you get a sad gray box of MrG's less precious items. Better luck next time!
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Like the loot boxes you find in a normal beanstalk these prep boxes are locked (remember you're looting MrG's preciouses). So you'll need either your own FFFF Key or the beanstalk's Stained Bone Key to open them.
There is a way you can create a Golden Egg yourself if you can't find one. You will need at least one piece of unused Inchoate Gold, a beanstalk's stained bone key, an emptied heart or crown loot box from the same beanstalk as the key, and a Barren Pinkly Egg.
You won't lose your key, it stays on crafting grid!
If your beanstalk has any hidden prep boxes available, you will see a single-use Golden Egg in the output slot of the crafting table. You can use this Golden Egg once to swipe a prep box from the linked beanstalk. You can repeat this process so long as you have pieces of Inchoate Gold to sacrifice. Just use the newly obtained box instead of the original heart loot box in the same recipe next time.