There are two ways to look at the selection of a pendant. You are either going to look first at the gemstone being used, and then at the design of the rest of the pendant, or you are going to be drawn by the design of the pendant, and then notice the gemstone. Neither way is wrong, it's all a matter of personal taste. Generally these two things will be the deciding factor of which pendant you really want to buy.
But there are some things that should be considered when making the decision of which pendant to purchase:
Size – before you choose a pendant think about the size of the person that pendant will be put on. The person's body will make a big difference in how the pendant looks. What is the overall size of the pendant? If it's a small, intricate design, it may not be flattering on a larger person. A small pendant will actually accentuate the size of the person, something they may not appreciate. Likewise, a large pendant may look a bit overwhelming on a small person. Not everyone can wear massive pieces of jewelry. Remember to keep the person who will be wearing it in mind as your peruse the options.
Attire – What will this pendant be worn with? If the person who will be wearing the pendant has a penchant for v-neck dresses and tops they will be perfect for most pendants. Pendants set on a thin chain tend to pull the chain down into a V shape that will match the neckline of their clothing. If, on the other hand, it is a person who likes rounded collars, you may want to opt for a thicker chain, or even necklace of pearls to give it a more rounded shape to complement the neckline.
There are a few other considerations to take into account when selecting the chain to go with the pendant.
When will it be worn? If it's an everyday wear necklace you want the chain to be shorter and not as embellished as other pieces may be. If this is meant for special occasions you may want to select a more elaborate chain, to really show off.
How old is the person wearing it? For older women it is better to give them longer chains for their pendants. Remember, the pendant will draw attention to it. If the chain has that pendant hanging near their neckline, and the neckline is less than perfect, it may be accentuating a less than desirable trait. A longer chain will draw the eyes away.