Ceramics are fired in a kiln at high temperature. This is one reason that most artists do not add items to their ceramic pieces. Objects such as leaves or rope are used to texture the clay before it is fired the first time. If they were left on the clay piece, these items would burn while in the kiln. A kiln fire is dangerous and can damage the kiln. This is one of the reasons this is not usually done.
There are a few items that can take the fire of the kiln and survive. Gem stones and glass are two items that can withstand high temperatures. Glazes are actually mixtures of ingredients that will turn into glass when fired in the kiln. While not all gem stones can withstand high temperatures, there are a few. Diamonds are one stone that can take large amounts of heat without breaking, cracking or catching on fire. These beautiful stones are forged under high heat and pressure during their formation. That is the reason they can take the heat of a kiln.
It would be unreasonably expensive to add natural diamonds to many ceramic pieces as they are very expensive. Even diamonds made in a laboratory are relatively expensive and their cost might far outweigh the value of the ceramic piece. Stones, pebbles and metals are a way to add texture and color to ceramic pieces. These are added before the clay is molded. While some of these ingredients add texture, others are used to create new and different colors in the clay.
Artists continually explore their field to find new ways to create beautiful objects. Mixing a variety of materials into clay before forming it is still being explored. Changing the texture and color of the clay is the objective of these artists. They want to find new ways to create beauty with their ceramic pieces.