This is a cosplay prop of a blue diamond for the Blue Diamond character in Stevens Universe. Honestly, I've never seen the show but this was requested of me. The other ones I see on here all have a flat base but nothing I could find online indicated it had one, and the reflections are better with it cut so I felt this was more accurate.
It measures 3 inches high, 2 inches wide, and is 0.25 inches thick. I felt this was a pretty good size to get the same look on an actual human, but of course it can be scaled if you want to in your slicing software.
EDIT: I added photos of it printed in transparent blue PLA made by AMZ3D
I used Fusion 360. I made a sketch on the X/Y axis where I created a vertical construction line and a horizontal construction line and added a coincident to the origin. I then drew the shape while intersecting the lines with the construction lines (these act as constraints to keep it centered where I wanted). I then added parameters for the height (3 inches), width (2 inches), and height from the top where it is widest (2 inches). I also added a thickness parameter (0.25 inches). I stopped the sketch, then extruded symmetrically with a -70 degree angle to give the cut look I wanted and I feel most closely matches the reference material. That's it for the modeling, it's done! Easy, right? Feel free to try it yourself.