Some initial notes
Let's make a google search (images):
1) You can see how difficult is to make an abstraction of a crystal. Verey few logos succeed there.
2) "How to make an illustration" depends on what kind of illustration do you want. The key word here is "style".
Depending on what is this approach the look could be called a diagram, a cartoon, illustration, cute, amazing, etc.
Let's make another search (images)
That is better but the problem is that you don't need faceted crystals which are easier in some cases.
3) It is obvious you don't want this dull diagrams of lenses: https://www.google.com/search?q=lens+diagram
Here is my aproach
Actually I'm starting to use this style for some diagrams I'm doing myself.
If we need a realistic image I would use a 3D image as a base.
But if we don't need the realism, just a cuter diagram I'm using a simple shadow (s) and a reflection (r) (A)
But I can not see the reflection so I need some base color (B)
And I am adding a "sandblast" (C)
But the trick is that all those elements are actually using transparency. It is a lens after all. So If I use a darker background I know I can show light in the next steps. (You can see that the beam of light is behind the lens)
I can play with the light instead of those boring yellow arrows.
All transparencies are simple gradient ones (I'm using Corel Draw here but you have them in Illustrator and Inkscape)
For the last one I'm using a starting object and a final object to give shape to the beam, but there are other approaches to that.
Edited some time later.
I'm adding an image on how to make a simple typical glossy button.
You can see here how the different simple elements combine together.
An additional variation regarding style is adding some blurs on the same objects we had before.