This collection page at first seems surprisingly big- until you find out that it’s for the Mag Mell Ruins as well as Tephra cave. Then it... well, it still seems rather large. That said, there simply isn’t enough room in the Cave and Ruins together for all of the collection items to spawn- it’s likely that you’ll have to go back and forth through the cave for a while to gain a full set- though you’ve no reason not to. A set of six Ether Gems ranging from very nice to fairly good is nothing to sneeze at, and Warrior Cuisses are a useful piece of armour. Do make sure you take a look at the info entries on the Collection Book as well, though- some of the descriptions and stories on some of the items are rather amusing.
Fruit: Clear Almond, Bright Fig, Dark Grape
Reward: Lightning Attack II Ether Gem
Flower: Night Tulip, Insanity Mint
Reward: Poison Defence III Ether Gem
Animal: Shin Newt, Cave Rat, Shin Gecko, Happy Rabbit
Reward: HP Up II Ether Gem
Bug: Gold Caterpillar, Brown Butterfly, Rumble Stonefly
Reward: Slow Resist II Ether Gem
Nature: Kneecap Rock, Clarity Moss, Confusion Ivy
Reward: Attack Plus II Ether Gem
Strange: Leaf Mystery, Steel Silk
Reward: Attack Stability II Ether Gem
Page Completion Bonus: Warrior Cuisses