Hi Shanna thanks for writing in! Lucky you! Sounds gorgeous!
there is a simple test for diamonds IF the stones are loose, not set into the jewelry. You will not be able to do this test on your piece because the stones are pave set, but for future reference...
Draw a black line on a sheet of white paper and place the stone, table down (the top of the stone)on the line. If the stone is a genuine diamond the line will not be visible.
You mention the stones "behave" like diamonds. You have to be careful here because some of the diamond look-alikes on the market are so close to a girl's best friend, that even experts have a hard time trying to determine diamond fakes.
Yes, you are right absolutely, the hallmarks on the jewelry will help you determine the origin, and maker and age. However there are millions of different types of marks on jewelry, not all are maker's marks, the triangle you mentioned could be a duty mark for instance, so without a photo of the marks, in particular the marks that are hard for you to see, images of these marks are vital for us.
Have the stones verified as this will affect the value. Try and get some images of the jewelry, preferably three images (one of the mark - close-up)including the area around mark and one photo of the piece itself.