A cabochon is a gemstone has been shaped and polished, as opposed to faceted. The term comes from the French word caboche, which means "head". Cabochons are used primarily in jewelry making. Jewelers use cabochons to create jewelry that features different types of gemstones, natural and man made.
For beginners, I recommend a cabochon that has rounded edges. Ideal shapes are round and oval. I recommend something large sized and a durable material like obsidian, fossil, or crystal because it will withstand the force of bezel setting better. I do not recommend practicing your bezel setting skills on a soft material like amber or opals, it is more prone to being scratched or broken.