Answer:
I dig out tiny caves and store gold and silver in them. I also build bridges of silver and make crowns of gold. They are the smallest you could imagine. Sooner or later everybody needs my help. Yet many people are afraid to let me help them.
I'm sometimes white and always wrong. I can break a heart and hurt the strong. I can build love or tear it down. I can make a smile or bring a frown.
Turn us on our backs and open up our stomachs you will be the wisest of men though at start a lummox.
If you drop me I'm sure to crack. But give me a smile and I'll always smile back.
In we go, out we go. All around and in a row. Always, always steady flow. When we'll stop, you'll never known. In we go, out we go.
I repeat only the last word you say. The more I repeat, the softer I got. I cannot be seen but can be heard. What am I?