Yes I have used Gem Eye. The trick is to really really clean the stones your testing. I have tested them against the GIA refractometer I typically use in my Lab. You have to know your stuff though just like with the GIA one. It will tell you the to choices of what your testing and give you the RI as well as other possibilities. It won’t tell you if it’s synthetic or heated/treated in any way but there are test for just that.