27mm BIG Glass Heart Stone in Medium Siam! This is a medium, blue-based red. Beautifully faceted, and foil-backed to catch the light. No hole, but I do have the premium backless sew on settings for these in Imitation Rhodium and Gold Plate.
Very similar to the 28mm Swarovski #4800 hearts, though they don't quite fit those settings (which is why I hunted down settings just for these ).
You can find the sew on settings for this stone here: https://peacockbeadshop.com/products/psos-27-hrt-setting