Dolomite

It can occur in coral reefs, often near an underwater volcano.
Overview
Dolomite is a mineral that can be found in a Magma Geode and an Omni Geode. It can also be produced by a Coral Fish Pond once the pond population reaches 9. Like other minerals, it can be processed for tailoring and dyeing.
In the Sewing Machine, Dolomite is used with Cloth in the spool to create a clothing item, and it functions as a pink dye when placed in the dye slot with a dyeable clothing item. It can also be placed in the red dye pot at Emily and Haley's house, 2 Willow Lane, for use in dyeing.
Official description
It can occur in coral reefs, often near an underwater volcano.