silver "nugget" found

Was picking up my breakfast bagel from the grocery store this morning when I spied a little surprise in the coin star. In the return slot there was one 1941 wheat cent, one 2006 Euro Cent, one 1997 Roosevelt dime, and two silver 1964 Roosevelt dimes. Then I found in the coin tray up top one 2006...

i was metal detecting in iowa and had some strong hits on my detector and found 2 nice sized rocks that after I cleaned them resemble what I believe to be solid silver. hopefully someone can help me identify them. thx