I love the sparkle of crystals, and you can add them to your resin projects, but if you let the facets of the crystal sink below the resin, the light will no longer be able to reflect off of them, and your crystal will look like a flat-backed mirror. So, there two ways to keep that sparkle: 1) pour a super-thin layer of resin, so the dimension of the crystal sticks out above the resin, or 2) let your resin set up for 1-2 hours, so it's firm enough to support the crystal, but still sticky enough to bond. This can be tricky and you'll have to experiment a bit to get a feel for timing. I use a pin or the tip of my mixing wand to test the set. If you miss the firm/sticky window, you can also glue your crystals on after the resin is fully set, using ZapAGap and a pin or toothpick to apply.