a vein of silver的用法和样例：
- There is a vein of melancholy in his character.
- A vein of gold was seen cropping out.
- There is a vein of humor in his nature.
- A vein of copper was seen criopping out.
- Along the edge of the mountain there is a rich vein of silver.
- The mine once had a rich vein of silver, but it petered out.