Snowflake Obsidian is a naturally occurring volcanic glass. My sources tell me that it is formed when "Felsic lava that is extruded from a volcano cools without crystal growth." Which - of course - sent me off to my good friends, Google and Wikipedia, to find out what felsic lava is. It's very easy for me to get all distracted by the stuff I find on Google, but the skinny is - when rock is molten in the ground, it is called Magma. Once it leaves the ground - it is Lava. Lava comes in three consistencies - thin and runny (Mafic), thick and gooey, (Felsic), and some that is somewhere in between, called, less interestingly - Intermediate Lava.
It is the thick and gooey stuff that we are concerned with here - the Felsic lava - which has a higher content of silica, and when it cools rapidly - it becomes that glossy black stone we know as obsidian.
Snowflake Obsidian is the inclusion of small, white, radially clustered crystals of Cristobalite to give it the snowflake pattern. I looked up Cristobalite too - but honestly - did not get any answers I understood - except that it is named for Cerro San Cristobal, Mexico.
Obsidian is pretty cool stuff, and has been in use pretty much as long as humans have been picking up shiny rocks and hitting things, which is, essentially - always. It has been used as tools, knives, arrowheads, etc., since long before recorded history - as when it is prepared correctly, it is sharper than steel.
Metaphysically - it is said to calm and sooth, and teaches you to value mistakes as well as triumphs, to let go of unhelpful mental habits and promote dispassion and centering. It also is said to have many connections to "truth" - from use in scrying to bringing hidden truths to the surface in dreaming.