What makes 3D engraving different from standard engraving is the variation in depth. Some parts of this pendant are carved shallow, others a little deeper. This is controlled partly by the artwork, and partly by the laser's control software. The artwork itself is not just black and white, but also several shades of gray in between. The darker the shade, the deeper the engraving. The trick is to run the laser at low power and engrave in several passes.
Here's the result:
