If you hold a magnet near the gold piece and it sticks to that magnet, then it may be impure or some metal mixed with the gold. While you rub this gold against a glass piece if the glass gets scratches, then it is not pure gold.

Scrape your gold against the piece of an unglazed porcelain tile to check the mark made. If the mark looks gray or black, then the gold is not pure. If the mark appears yellow, it’s pure gold. You can file some small section of the gold with a metal file. If the gold is pure it will get filed relatively easily.

You can do an acid test using the hydrochloric acid. Using a cotton swab dipped into the acid, you should drip a small amount of hydrochloric acid on the filed area of the gold. If it starts breaking down, the gold is not pure.