Have read through this topic with interest. Here are my thoughts. I've shared similar before, here, but don't remember the name of the thread, etc.
First - if you want to work with a pendulum, it doesn't have to be made from a crystal. Your can hang a nut or washer (yes, I'm talking about hardware here) on a string and use that. Or pick something prettier, like a piece of jewelry. I do sometimes wonder whether people muddy the waters of their attempts with pendulums, by making them from crystals, as then you have added another energy to the situation. The power of a pendulum is not (not not NOT!) in the object itself! However, some very interesting energies do reside in crystals. If you are adept with both crystals and non-crystal pendulums then you might find a way to have the two working together for some kind of added effect. (And to all you who have only ever worked with your crystal pendulums, I say "well done" and that's fine - as long as it is working for you. However, I believe you are probably giving the crystal more credit in the deal than it is earning.)
Second - I have occasionally had pendulums do the trembling thing. I'm pretty sure it has to do with my own energy state - both mental and physical, which are related obviously. Try doing some experiments.
- Try holding the pendulum differently, or in the other hand. If you normally prop your elbow on the table, don't. If you don't normally prop your elbow, do. Try standing, sitting, changing your breathing, etc.
- Try a different pendulum. Not buying one, just make one out of something. If you get that working, go back and try the one you're having trouble with.
- Try swinging the pendulum on purpose. Forward and back, side to side, in clockwise and anti-clockwise circles. Get to know its weight and feel. It's possible that this is what doesn't suit you. Some people prefer more or less weight or a longer or shorter string/chain. The weight of the pendulum relative to the length and thickness of the string/chain can also affect things.
Best of luck!