Dark abalone shells tend to have a darker blue interior.
Abalone shells make excellent vessels for containing smoldering herbs and incense. It is wise to insulate the bottom of the shell with a minimum of 2 inches of sand or ash to protect the shell against high temperatures. These shells, used for traditional smudging, come from Mexico. They are ideal for burning sweetgrass and sage or indirect burning incense such as resin.
Place a few dried sage pieces or sweet grass in the bottom of the shell with the sand, ignite then blow out flame and the sage or grass will release a cloud of white, purifying smoke.