Instructions
Plant a gem tree seed in your garden. If you do not have a gem tree seed, buy one from the shop for 238 coins.
Wait for the tree to grow and mature. As it grows, it shakes off some of its leaves and buds begin to form.
Go to the store when you see the gem tree shake off its leaves and form buds a second time. Buy the blue fertilizer.
Apply the blue fertilizer to the gem tree immediately. You do not receive a notification that the tree was fertilized, but when the tree's buds open, you receive more gems. Gems also grow back faster.