You can make heart gems in alchemy bt you have to use three really high star rated food like diamond fruits as long as they are three star food with high value you should get heart gems then mix the heart gem with other high quality foods the higher the quality the more hearts you receive.

I have been trying to

Auto correct changed heart gems to great gems

You can also get hearts with great gems by using alchemy usually takes three really good items like the star items ex: diamond fruit. Crystal donut. Magic gourds

I went to half training bar then fed it alot if you keep your lights off I.found they will not evolve till the light is on and you can still fed them and everything