Stacking is easier than it looks. A few simple rules do most of the work, and they apply whether you own three pieces or thirty.
Pick one statement piece. Choose the piece you want noticed, like a moss agate bracelet or an opal necklace, and let everything else support it. Two statements side by side compete.
Vary the scale. Pair one bold piece with finer ones. A slim ring next to a larger stone, or a delicate chain under a pendant, gives the eye somewhere to rest.
Keep the metal consistent. If you are wearing yellow gold, keep the pieces in yellow gold. Mixing in white can work, but do it on purpose, not by accident.
Layer necklaces at different lengths. Adjustable necklaces help here. Several of ours fit 16" to 18", so you can set one shorter and one longer without a spacer.
Repeat a detail. A shared stone, a shared shape, or a shared setting ties a stack together. Many of our rings and necklaces share a diamond-shaped setting for exactly this reason.
Start with one piece you love, add one, and look in the mirror before adding a third.