Quote from Frawmus;3174:
Its luck, but higher level= bigger chance of success which equals more chances of getting a gem?
yes.
"You are able to find any Ore Fragment or Jewels (gemstones) at any rank. A higher rank merely increases the skill's success rate."
"Jewels... will appear at random in conjunction with a success."
in other words, the higher the rank, the higher the chance of success = getting ores. Every time you get ores, you get a chance of getting a jewel. so in essence, yes, the higher the rank, the higher the chance of getting a jewel.
ps. i highly doubt the time of day affects the size of jewels. i've metallurgized for a long ass time and i've never seen any correlation between the two factors. and i've capped metallurgy with rank 1 prod.