Basalt Monolith, Tough Cookie, and Heartstone

Prerequisites

• Basalt Monolith on the battlefield
• Tough Cookie on the battlefield
• Heartstone on the battlefield
• Basalt Monolith does not have summoning sickness
• available

Steps

1. Activate Basalt Monolith's first ability by tapping it, adding
2. Activate Tough Cookie's first activated ability by paying , making Basalt Monolith a 4/4 artifact creature until end of turn
3. Activate Basalt Monolith's last ability by paying , untapping it
4. Activate Basalt Monolith's first ability by tapping it, adding
5. Repeat from step 3

Results

• Infinite colorless mana