I was on the way out of detecting in an old county rodeo site when the morning sun caught a clear stone's flash. It is almost like yours. Back then I didn't have a diamond checker, so I took it to an old jeweler. He taught me one way to tell if it was either a diamond or just a CZ.
If you can find some very small fine print from some old assembly instruction sheets, or found small lettering around old or foreign postage stamps, turn the stone upside down so the facets are facing upward. Slowly move the clean dry stone over the fine print. If you can see the printed letters move as you look through the stone with the help of an ordinary 3X to 5X magnifying glass, it means the stone is most likely a CZ. A diamond's facets reflect so well inside the precious stone, the back view looking through the cut facets is almost impossible to see through.
The one I found was a large CZ. But, because I find 10 times more gold, silver, and some diamonds at garage sales than metal detecting, I bought a battery powered "thermal" diamond tester. It's made by Tri Electronics Inc. and called " DIAMOND PRO DUAL TESTER". It is made to tell the difference between metal setting itself, CZ's, Diamonds, and Moissanite stones. I've never found Moissanite, so far... ;>)