Traditionally, we are taught to burn clearing plants and herbs in natural holders.  Place your sage, palo santo, or other herbs in to clear yourself or your sacred space.  Spiritually, the abalone shell is considered a gift from the ocean which represents the element of water, your smudging herbs represent mother earth, when you light your herbs you are bringing in the element of fire, and your smoke represents the element of air.  Balance of the four elements is very symbolic.  Each shell is unique and does not look the same as they come from the ocean.