Diamonds will definitely shatter. They are crystals and have cleavage planes.
They will not be scratched by a file or corundum (ruby).
The easiest, best and most certain way to test is via Index of Refraction, but that's not possible without a specialized oil or a testing machine.
Usually, the setting is a good indication of whether it is a valuable stone or not. Rare is the diamond in a silver setting/ring.
A good way to test for natural vs manmade stone is to closely examine it under the highest power magnification you have available. A natural stone will virtually always (99.99999999999999999999999999999999999) have flaws in it such as inclusions and obvious cleavage planes. This works for all gemstones.
Best thing to do is take it to a jeweler, coin, or pawn store. Be laughed at- what do you care? If you are close to me, I'd be happy to give you my opinion.
I didn't stay at a holiday inn, but I am a geologist and lifelong rockhound.