Opal and Tourmaline are the birthstones
for those with October birthdays.

Tourmaline comes from the Sinhalese word for mixed colored stone, tura mali. In fact, tourmaline is not a single mineral, it’s a group of minerals with very different chemical compositions and colors. 

A tourmaline is a boron silicate mineral that occur within igneous and metamorphic rocks.

Your second birthstone is opal, which gets its name from the Greek word opallios. This roughly translates into "to see a change in color" as demonstrated when you spin an opal.

Opal forms when water picks up silica dioxide and deposits it into open voids or cavities along with trace impurities. The water then evaporates and leaves the silica dioxide. Most of the opal in the world is produced from Australia, where seasonal rains make it common for water to leach out silica and deposit it into cracks within the ground.