These pearls have a Pilot Hole drilled half way through. This makes it easy to mount a pearl to jewelry.
The Pilot Hole size is 0.7mm
What is a Pilot Hole?
A Pilot Hole is a starting point for drilling. Typically a small hole will be drilled as a guide prior to drilling larger holes. This prevents drilling a hole larger then is needed. A Pilot Hole also allows for drilling holes larger much quicker as the hole is already lined up.
You may need to bore the hole out slightly larger to fit larger mounts. Since the pearl has a Pilot hole this will make the process much quicker.