Although you need 5 flourite for the renovation projects (and maybe more for additional maker sheds or stuff) as opposed to only 4 total platinum (as far as i know) for the ultra axe and hammer. So it's up to you i think.
The rarity of an item depends on how badly you need it. The game has a built in sensor that can read your very thoughts and desires. If you want an item, it will make sure that item is practically removed from assistance, no matter how many times you reload.
You can get fluorite from the blue spot, and you can get platinum from the red and green spot.
Add the save exploit, fluorite becomes harder to get, although platinum has more uses and value.
Flourite was my nightmareXD It took me near to the end of fall to finally get the fourth renovation (Now almost finished besides the three waiting tasks of potatoes/yams, garden festival, and intermediate class with vegetables and/or otherwiseXD) I got platinum sooner than flourite but maybe that was a luck of a draw (since I soft save regularly to reload if things don't go well)
So the problem with Fluorite is, as it's only blue, if I mine a recipe I keep it. And fluorites doesn't like me, and blue has a high rate of recipes
Meanwhile, the green & red gives platinum, and it has a low rate for recipes.
So I say, I prefer fluorite.
I have mined over 20 fluorites in my time and only 2 platinums. It is a your mileage may vary thing. So I would wait to trade something until you figure out what you seem to excel in.
Let's see.. I've found 4 pink diamonds, 14 platinums, and 7 fluorites (which 4 I reload because I was looking for amethyst at the time)
Yeah, it always makes you feel that whatever you need is rarer. Except rocks :p
platinum for me was so hard to get when I was trying to upgrade the hammer , I still only have a few of them . In my game I think the platinum is the rarest besides the diamond but I have 3 now