Prototyping implies NOT making incredibly expensive molds to produce small quantities of lenses. Plastic lenses change their shape while cooling which implies controlling your process very carefully to achieve predictable results. There are a whole lot of different glasses - each with differering index of refraction and dispersion available. Dispersion (often referred to the Abbe number) and index refraction are how optical materials are specified. Back in the bad old days Abbe mined Calcium Fluorite to combine with normal glasses to make the first Apochromatic microscope objectives at Zeiss. He and Carl Zeiss invested in the startup of Schott Glass because having more glass options was important to getting where they wanted to go at Zeiss. The lasers we use for light shows today are essentially a single color it's possible to do a lot with a single lens - that's not the norm in optical design.
"There are painters who transform the sun into a yellow spot, but there are others who, with the help of their art and their intelligence, transform a yellow spot into the sun." Pablo Picasso