This will only work for devices that have a magnetics sensor. Calibrate you phone first by holding it upside down and moving it in a figure of eight. This should reset the reading to below 100 when metal is not close. Use your phone's built-in magnetics sensor to detect metals that distort the natural magnetic field.
This download is completely safe. Dr Quiz is the developer of this application which can be found in the Tools category. According to the latest info, the app is rated 3.6 on Google Play.