Tell me, what makes the stones powerful? What do you believe resides within them that causes them to be capable of such magick?
In my beliefs (animism) everything has a spirit. Does that mean every one of your beads is a sentient being? Well no. But it means that each is a piece of one. We all know most gemstones are "mined" from under the earth. For this reason, some even call rocks, stones, and crystals the "bones" of Mother Earth. Powerful thought right?! So if these stones you have are the same idea of the bone relics of a saint, but instead the relic of the very land it came from and the sentient being within, then what is "powerful" within them is not them but what they are a connection to.
Now what does that mean to you? That is what is important. Personally, I would sit down with these stones and use them in meditation to connect to the earth. Then, I would take each type of gemstone and meditate with it separately in order to sense what aspect of the earth it had to bestow. Stones and crystals all have different properties because they are made of a mixture of different properties with different natural environmental mixtures of heat (fire), moisture (water), and gas (air)...whoa, wait a minute! I didn't..yep, I did. Now you see where the other elements come in? Hehehe.
So now that you know your stones on a personal level. How do you get them to really work with you in this necklace to help you out? By the time you are done learning from them, you may not need me to tell you.
Personally, I like to bless them in smoke (air). This can be as simple as lighting fire to a few bay or sage leaves and letting the stones pass through the grey plumes. This is using another part of Nature and the bay or sage should be equally respected and once burned disposed of in the ground not the garbage.
Then I would spend time meditating on what I want the stones to focus on. I would visualize it, whisper it, maybe hum, sing, or chant, and feel it like a memory yet made while making the necklace so that I could literally build it with my willpower involved. Creating a spell as an object is an ancient practice.
Good luck. I hope I helped.