What device is used to hold a lens in place during the edging process?

The device used to hold a lens in place during the edging process is a "Block." This tool is essential in opticianry because it secures the lens to ensure precision in shaping and finishing. The block typically adheres to the back of the lens, allowing it to be properly aligned and held steady while being worked on. This is crucial for achieving the correct curvature and proper fitting of the lens into the frame.

In contrast, other options do not serve this function. A Blocker is related to the process of squaring or preparing lenses but is not the actual device that holds the lens in place during edging. A Colmascope is used for inspecting the quality of lenses but does not play a role in the edging process itself. A Vernier Caliper is a measuring tool used for checking dimensions but does not hold or secure lenses. Therefore, "Block" stands out as the correct answer because it plays a directly integral role in ensuring the edges of the lenses are handled accurately during the edging process.