Measurements: One Size fits most.
Imagine standing on a high cliff, the ocean beyond you. The air is cool and pure. You feel as if you could see forever. Seagulls swooping, pelicans propelling across the sky, and a ship edging across the far horizon are all equally sharp and clear. Remember putting on your first pair of glasses and discovering that trees had individual leaves on their branches and the lawn was made up of blades of grass distinct from each other? If something is crystal clear, it can't be any clearer. That's the kind of clarity rock crystal can give to our perceptions.