I'm prefacing with a warning: If you've never done something like this before, or are at all less than absolutely confident in the process, only do this with someone you trust, preferably more experienced than you, at hand. I learned the seeds of this one in a group to which I used to belong; most of us were fine with it, but one or two wound up in small puddles of tears afterwards with a lot more to work through after than before - and that was having gone without a target in mind. If you've got one, it could be all that more acute.

This is the framework of a simple "guided meditation" which can be expanded into journey-work; I'll keep it as bare-bones as I can so any embellishments are your own.

After descending into a meditative state and meeting your spirit helper (who should accompany you throughout), find yourself on a path on which you walk to an entrance to the ground. Go into that entrance and explore. You will find yourself in a cave with many colored stones on the wall. One will draw your attention - touch it. You'll find it falls into your hand. Exit the cave with the stone. You find yourself back on the path under the noonday sun. The path will take you to a clearing. Sit under the sun and examine the piece that came with you. As you examine it you'll find its facets and the light reflected through them will mesmerize you - and then the rock will be gone, and the maze of facets and light you contemplated will be a part of yourself, returned to you. At this point, feel free to leave.

Now, a few clarifications:

1. Make sure you have a spirit helper with you - it's very easy to get tricked into bringing something back you don't want.
2. The cave we're examining here is NOT the classic cave into the OtherWorlds, but one into yourself. For clarification, see my discussion on spirit walking versus shadow journeys to discuss the two. Given the language of my own path used here, I'd go into the cave for a shadow journey to get my piece back, then go back to the grove - but not beyond - for the examination and absorption. I would advise using similar constructs, if you have them.
3. When you go in the cave it's entirely possible that, depending on how your mind works, you might find an entire network of tunnels. Be careful not to get lost, and if it looks like you might, take that as a warning that you're Not Ready Yet. The "yet" part roughly translates to "do the work" to get ready, because you could do yourself damage otherwise (example: going hunting for what you feel is a lost piece of yourself, and coming back with memories of abuse - perhaps you suppressed it or maybe it's a fabrication, but now, when looking for lost bits, is certainly NOT the time to deal with it!).
4. Coming out to the sun and someplace you feel safe is very important - if you're going to open yourself like this, you want protections around you.
5. I would advise doing this whole thing within a circle or other purified area on the physical level.