the problem is not lack of light. the problem is too much as its causing shadowing and glare. try this
1. if coin is in holder, remove coin from holder if possible
2. turn off flash
3. place coin on flat surface with indirect natural light. maybe a window sill in a cloudy day. Use a black or white background to avoid any reflective color
4. move phone about 3in from coin
5. zoom in using your fingers until the coin comes into focus and fills the cameras view finder. if your phone has a portrait selection use that and then press and hold on the coin in the view finder to AF lock it
6. hold phone steady. if you can't then place something on either side of the coin (like books) at that distance and lay phone between them. this will hold it steady and allow you to zoom and take the photo without any blurring