The in-law is very skilled in examining diamonds. He has accurately identified the truly perfect diamond.

This beautiful couplet, often sung at weddings, uses a lovely metaphor. It says that the in-laws, specifically the bride's father, are experts at judging diamonds. And with great confidence, they have chosen a truly perfect diamond – a flawless gem! Here, the 'diamond' is actually a heartwarming way to describe the groom. It means they've found an excellent match for their daughter, someone truly valuable and worthy, and they are absolutely certain of their choice. It's a sweet way to express approval and joy for the new union.