This is a very rewarding project, as it harkens back to the discovery of the terrarium, when Nathaniel Bagshaw Ward put a fern in a bottle.
Many of the pendants you see on-line expect us to make do with little curls of moss as the only source of green, but you can find plants such as Neoregelia lilliputiana that aren't supposed to get bigger than the volume of your bottle. If you are willing to maintain your pendant by pruning or replacing the plant when it starts to outgrow the tiny space, you have a lot more options. Keeping the plant in such limited space will slow its growth, so pruning should not be something you have to do often.
I find that a number of cacti and succulents make excellent terrarium necklace specimens. Plus, you can consider it part of the spring cleaning in your greenhouse, as only the tiniest plants will fit in the bottle. Look for offsets that your favorite plants may have produced. (Cactus offsets that don't have any roots yet should be allowed to sit for about a week, to allow the bottoms to callous over, as they tend to rot when placed in soil while still damp.) If you don't find offsets on succulents such as sedums, you can pinch off pieces of the plant and attempt to root them directly in the pendant.
Because I used a cactus for the sample pendant, I gave it a rustic, deserty look, using western-theme dangles for an extra touch. You could create a more elegant look, with satin cording instead of leather. Be creative.
A small bottle (preferably less than 2 inches tall)
A cork (that fits that bottle)
1 spoonful gravel (the kind that goes in fish tanks is about the right size)
Activated Charcoal (you only need a few pieces)
A small bit of spanghum moss (optional)
1 spoonful potting soil
1 small plant (cactus, succulent, decorative moss, or anything else small enough to fit in the bottle - choose something that doesn't need huge amounts of sun)
A 18" long piece of 24 gauge wire
16" of leather thong
You will definitely want a pair of tweezers and sharp-nosed cuticle scissors to manipulate your materials as you place them in the bottle, as well as a spray bottle filled with clean water. You will also need a wire-cutter and a pair of needle-nosed pliers.
PREPARING THE TERARIUM ENVIRONMENT
Approach this project like you would any terrarium, only think tiny. Use the same layers of materials - gravel for drainage, topped with a thin bit of activated charcoal to keep things fresh, topped (if there is enough room in the bottle) with a tiny bit of moss to keep the dirt from filtering down into the rest of the layers, and finally soil for your plant to root in. You may need to break up the charcoal (but do not powder it, as this will just sift down through the gravel, and try to use soil that doesn't have large pieces of mulch.
Carefully push the plant (root end down) through the neck of the bottle. You may have to apply gentle pressure with your tweezers as you do so. Use the blunt edge of the cuticle scissors to manipulate the plant so that it is sitting upright on the potting soil, and, if necessary, to move the soil to better cover the plant's root. When you are happy with it, spray down the sides of the bottle with a little water to remove loose dirt and add the moisture needed for the ecosystem. Cork the bottle.
MAKING IT A PENDANT
There are several ways of turning the bottle into a pendant. One is to put an eye hook through the cork and thread a chain through it. It takes a little more effort, but I prefer the look you get by wrapping wire around the neck of the bottle. This also puts less stress on the cork, which can be fragile, and will have to be removed occasionally to add water to the pendant. Make a loop at either end of the wire before wrapping it, and attach the leather thong to the loops.
Remember, the plant inside your pendant is going to need sunlight, so don't store it in a jewelry box. Also, try to keep it sitting upright when you aren't wearing it.