Yes.. Corrosion! Some tumbling around in a river with lots of pebbles will wear the features down also.
How much does it weigh? If it weigh's close to 5 grams, then it's not acid. If it's acid it's probably around 4.2 grams.
An inexpensive digital scale is a very useful tool. Coins, jewelry, gold etc. Try Amazon. Here's one similar that I use. Mine doesn't have hundredths of a gram, this one appears that it does. It looks like it even has thousandths. My needs are simpler and tenths are enough.