When gifting gem to NPC, how does gem size affect prize

The relevant NPC pages are collecting data on what "prize" the NPC may return if gifted an appropriate gem. They're recording gem sizes in a format like "3cm+", suggesting we believe that a particular prize might arise for any gem of 3cm or more. Unless someone found some docs saying this is true, I think we should also be recording an upper limit, being the largest gem that got that prize. The upper limit might be lower than 10cm.

Each NPC seems to have 2 possible responses to receiving their preferred gem. The responses can be quite long, so the quotes below are just snippets. I'm showing the size of gems that have triggered each response. I don't have useful responses for diamond yet, or indeed all relevant NPCs.

• Ethna - Jasper
  • "It's smaller than I hoped but still precious. 1.11cm - 6.00cm.
  • "Jeez, look at this size!" 8.00cm - 10.00cm
• Simon - Ruby
  • "Oh, look at this small yet beautiful gem," 5.00cm
  • "I've never seen such a big and beautiful ruby before." 9.00cm - 10.00cm
• Jennifer - Garnet
  • "Is this for me?" 5.00cm - 6.00cm
  • "Wow, this gem is really big." 9.00cm - 10.00cm
• Voight - Emerald
  • "It's smaller than I hoped." 5.00cm
  • "Oh my, where did you ever find such a big emerald?" 7.10cm - 9.00cm

So, my first guess is that there are only 2 size ranges of relevance in determining the possible prizes. The borderline is somewhere in the range 6cm and 7.1cm. So, we might find each NPC has two separate prize lists they choose from, one for big gems, one for small. Except that some items have been received with both small and big gems, so maybe the "big" list is a subset of the "small" list. The only way we'll manage to verify this - or prove it wrong - is if we record both the smallest and largest gem that gave each prize.

Anyway, I'm a little overloaded with gems it the moment, so I'm going to gift a few more and see if I can narrow down the ranges. I'll update the above numbers with what I find.