Can't tell value because we don't know the weight, color,
clarity. If it is a diamond, it has some value. You need to be a lot more detailed in the description or seek some outside, expert advice.
Something THAT pretty should not only have value but also attract quite alot of interest (such as mine). Any jeweler can help you determine weight, guess teh color range and measure the thing. Of course, nothing can replace appraisal and certification, but, whatever is easiest. I know that such a cut would be fashionable now. And you definitely made me (at least) curious! Such a stone is definitely my thing, althrough I am not sure how many would follow my taste
You know, this is actually appears to be a nice looking stone.
Just for fun I'll take a stab at an educated guess. Don't take it to the bank, because a guess is all it is.
The ruler and incandescent lighting appears to be throwing some color into it, but I'll bet it's at least a GH body color or better.
With the sharp corners as opposed to beveled, the chances are that it's an older modified emerald cut. There were a lot of well made sharp-cornered emerald cuts like this fashioned in the 20's and 30's, used in some nice Deco jewelry.
The clarity looks decent as well, with nothing obvious jumping out of the picture (although pictures make for poor clarity grading). I'll bet it's an eyeclean stone, probably VS or better if it is from the 20's or 30's. They tended to use better clarity goods back then.
The ruler seems to indicate approximately a 4mm width, and if the stone has a decent depth (which it appears to have), it probably weighs in the 1/3rd carat neighborhood. It looks to be nicely proportioned.
Let's say for theoretical purposes you've got a 1/3rd carat, GH/VS sharp cornered modified emerald cut with good proportions. Your appraised value would probably fall in the $750-$800 neighborhood, with internet sale pricing falling in the $450 neighborhood.