The physical term used to describe a lens material density is specific gravity. Most opticians know it as lens weight. The higher the specific gravity of a lens material, the heavier the lens will be. When choosing a lens material, you need to balance its thinness with its resulting weight. Weights of high-index materials vary considerably, with glass products leading the list as the heaviest. You might try to reduce a lens’ weight by using a higher index material, but often, as the index goes up, so does the material’s specific gravity.
An aspheric lens design is a good way to make lenses thinner, which also makes them lighter. What you need to determine is what the final weight of the lens will be using various index materials. This is the only way to be sure you are providing the lightest lens product. Unfortunately, this is not something you can easily calculate at the dispensing table with a chart, pencil, and paper. A good computer program is the best way to get this information.