This is a Selenite Specimen from Naica, Mexico. Naica selenite is some of the best in the world. It is well known for its high clarity and well formed specimens. Most of the selenite that you see on the market is actually satin spar gypsum, a related but different mineral. True selenite is almost glassy in appearance, like this piece here. Selenite is a very soft mineral, only coming in at a 2-2.5 on the Mohz hardness scale and will sometimes dissolve in water as well.
It is said to cleanse and charge any other stones placed around it and clear blocked or stagnant energy from people, places, or objects. It is a very popular stone for reiki and energy practitioners.
This piece measures approximately 16.3cm x 12.8cm x 9.9cm