An instrument implies something more delicate or technical than an implement, or alternatively something used for measuring. Eg. dentist’s tools could be instruments, so are for example the instruments in a plane’s cockpit, whereas something like a spade or a rake would be an implement. Also, as abhi pointed out, ‘implement’ can be used as a verb while ‘instrument’ cannot.That’s the way I’ve always interpreted it anyway, I don’t think there are any hard and fast rules.