I checked in-game materials inventory and the material trader. There are some oddities and discrepancies there.
Checking inventory we have:
Grade 5 - Antimony and Polonium
Grade 4 - Ruthenium, Selenium, Technetium, Tellurium, Yttrium and Zirconium
Grade 4 and Grade5 materials have the same cap, 150 units. However, Zirconium (G5) has a 250 cap - same as a Grade 2 material
Grade1 materials have a cap of 300, but not Boron (G1) which has a cap of 200
Yet, the material trader is splitting the Raws in 7 Categories. All the top grade in each catgory is Grade 4, there are no Grade 5s
Also Zirconium appears as a Grade 2 in Cat.7 and Boron is Grade 3 in the same Cat.7
So kind of a mess between in-ship inventory and the material trader - mess that might be reflected in missing Selenium from Crystal Shard - unless it was a deliberate omission and the mess above is just the typical FDev at work.