This is a wonderful estate ring set in an angular Art Deco style setting.  The stone is a synthetic aquamarine that is scissor cut (a variation of emerald cut) and set into a 10 carat gold setting.  The inside of the ring is stamped with a makers mark which is a script letter "f" inside an arrow shape along with the mark "10K".  The shoulder of the ring is a stamped geometric ridged design with three lobes graduating in size toward the stone.  In very good condition, the stone measures 5/16" by 1/4" or 8mm by 6.5mm and the ring is a size 5 3/4 to 6.

 

View of the shoulder.  Note that the brownish color

 in the stone is a reflection.  The ring's stone is a clear pale blue aquamarine: