Where did you find it? If you want a definitive answer, you could take it to a rock and mineral shop. Please take another picture in bright white light if possible. Avoid the color light used in the above pictures. Tell us what you know about where the rock was found like - on the shoreline of a salty lake in Salt Lake City, Utah. If you shine a light behind it, backlighting, does the rock allow light through(translucent) or does it block the light(opaque). Is it soft or very hard. You can tell by trying to scratch it with a metal file or a common nail. If it marks your specimen it is relatively soft, like limestone, if it cannot, it is relatively hard like quartz.