According to the wiki...Base Summon (2k summon)
Estimated probability of each rarity dropping from 25,000 trials:
Iron: 56.87% +- 0.81% with 99% confidence interval
Bronze: 40.69% +- 0.80% with 99% confidence interval
Silver: 2.38% +- 0.25% with 99% confidence interval
Gold: 0.05% +- 0.04% with 99% confidence interval
Platinum: Less than gold
Black: Less than platinum
Base Summon SP (Piece of Crystal x5)
Estimated probability:
Iron: 33.4%
Bronze: 41.9%
Silver: 16.3%
Gold: 5.0%
Platinum: 1.2% or less
Black: 0.3% or less