Very nice find. The color and clarity of the stone tells me it is probably some form of topaz. Most unusual by ring design and stone type. It`s not a quartz family member like citrine or Amethyst, it appears of the topaz family. I would rule out tourmaline family stones as well. What else this may be is a beryl family stone (emerald is a beryl but this is no emerald, but beryls come in several colors including yellow, green, blue even aquamarine amd stones shading in between these colors. I would definitely test it on a MOH`s hardness scale to narrow down what types of stone it may be.You may very well be suprised that this stone is quite valuable.