Got mine on the very first 8☆ Encountered. With my spouse. We broke and always break these 3 parts. #1 Tail #2 Head #3 Fin
Its honestly luck.
On average from my test ive ran breaking parts for R6 without breaking the specific part that tends to up the odds of it being dropped. About 72-78 Slays on fornfirst R6 did yhis for my wyvern gem shard ( emerald) on jagrass.
72-80 for monsters with special R6 (zenogre and devil jho) before i was rewarded.
Now that i focus on the part that drops it and get as many breaks as i can get (3 or more) i see R6 much more frequently. Try spamming tail break and snatch two more breaks too may help with drop. Patience is needed