Crystal cases, flowmass and flower caps (pre-2014) are made of a durable polyurethane (like skate wheels) and can handle over 140 lbs of tension on the leash. However, they can yellow over time and especially with exposure to sunlight - this is especially true of older crystal cases. We made improvements to the material in 2010 and they are much better in this regard, but they do still naturally yellow with UV exposure.
If they are just dirty from dust and dirt, we clean them in the laundry wash (not dryer).
If you would like brand new looking crystal cases or flowmass, please contact support@flowtoys.com - we will upgrade your old parts for half price under our forever flow lifetime warranty.
We are continuing to research material alternatives, but it has been very difficult to find a material that is strong enough, soft enough and clear enough that does not discolor! (and that is not vinyl, which is a terrible plastic for our health and environment that we choose to avoid.) We included a tint in one of our old batches to minimize the yellowing and it weakened the material significantly, causing many cases to tear.
Our newer products like the flowcap and pod shell are made of silicone, which does not yellow in the same way as polyurethane, but is a lot more expensive.
Also, we updated the v1 crystal cases to v2 cases which are 3D printed and will not yellow over time.
If you'd like to check out the updated cases you can find them on our website here: https://flowtoys.com/crystal-case-v2 and you can find flowmass here: http://www.flowtoys.com/flowmass-pair/
If you have any questions, please feel free to contact us!