Beware, unthinking youth

Puzzle

My first is found in many mines;
And there my costly second shines.
As for my whole—what shall I say?
It seems intended to betray.
Then, oh! beware, unthinking youth...
Adhere to honesty and truth.

Hint

Answer

References

• Loyd, Sam [1914]. in Loyd, Sam, Jr.: Sam Loyd's Cyclopedia of 5000 Puzzles Tricks and Conundrums (in English). New York: Lamb Publishing company, page 62.