The sign for Wales is to hold up your three middle fingers bent slightly forward (like a claw), and move your hand straight downwards.
Video here:
https://www.signbsl.com/sign/wales
The sign for Rose is the same as 'flower' but you use the lip-pattern (mouthing the word) rose, the sign is put your finger and thumb together and touch either side under your nose, like smelling a flower.
Video here:
https://www.signbsl.com/sign/rose
The sign for diamond is your non-dominant hand held flat, palm down (As if you were going to show someone your ring) and your dominant hand on top of that hand, fingers together and then open and close them quickly, as if to show sparkling of a diamond.
Video here:
https://www.signbsl.com/sign/diamond
There isn't an actual sign for platinum you would fingerspell p-l-a-t-i-n-u-m. You could use the sign for metal with the lip-pattern platinum, the sign for metal is your index finger taps your chin twice.
Video (for metal) here:
https://www.signbsl.com/sign/metal
Hope this helps