They occur as secondary minerals filling in the vesicles in amygdaloidal basalt, and in seams running through basalt. They formed from the reaction of the hot basalt with sea water when Guam was underwater. These are easy to find in rocks from roadcuts between Agat and Umatac. Large crystals can be found at Facpi Point. They vary in color from white to colorless to pink.
Some crystals are blocky and cubic, and others form radiating clusters of needles.
It is a component of basalt, but the easiest place to find it is where it has weathered out of the basalt and has accumulated as sand on beaches. It is the easiest of all minerals to identify because it is attracted to a magnet. The best place to collect this is on Talofofo Beach. Olivine is a primary igneous mineral that crystallized from basaltic magma. It is a component of basalt, but the easiest place to find it as in beach sand.