Voltaic Construct, Coveted Jewel, and Karn, the Great Creator

Prerequisites

• Voltaic Construct on the battlefield
• Coveted Jewel on the battlefield
• Karn, the Great Creator on the battlefield
• Coveted Jewel does not have summoning sickness

Steps

1. Activate Karn's first ability by putting a loyalty counter on it, causing Coveted Jewel to become an artifact creature until end of turn
2. Activate Coveted Jewel by tapping it, adding three mana of any one color
3. Activate Voltaic Construct by paying , untapping Coveted Jewel
4. Repeat from step 2

Results

• Infinite colored mana