I spent a happy hour picking Rainbow Obsidian Pillows as ovals, tears, hearts and more on a business trip. I was making sure each one had rainbows. Here is a lovely group. In the photos you may/may not see a rainbow but they are there. Also unfortunately black always shows up dust! There maybe minor inclusions on the backs. And for some the backs are plain black, or grey, or golden or purple, …. I have three sizes in the photo. Dimensions are for the larger.
Its a lovely way to add love to your learning about life which can be very challenging at times. This crystal is useful when used to help soften and ease the effects of shock or fear. And it can help protect the overly sensitive against depression. However they often are bought by people who just find these hearts decoratively interesting. Great birthday gift.
Rainbow obsidian pillows are a smaller shape so you can carry them or put in a hemp necklace. I will intuit the one that meets your needs.