Calcite is one of those minerals that can be baffling to a beginning collector, due to its myriad of different forms and colors. But it is a mineral well worth knowing -- fairly common and usually inexpensive yet lovely.
The calcite here is what is often know as "dogtooth" calcite after the shape of the crystals. The reddish color in this case comes from inclusions of hematite.
This particular specimen is from Santa Eulalia, Chihuahua, Mexico.