My and Turtlescanwrite 's theory is that Homeworld is at war. The Kindergarten is where gems made other gems for war.
Yellow Diamond sent Peridot to reactivate the Kindergarten. The only reason they would need the Kindergarten is if they were at war again. The bubbled gem in Lion's mane is bismuth. Bismuth is a metal. What is the Homeworld is at war with the metals.