Aquamarine and Tuff (white lava) in a 22 inch hand-knotted necklace.
Tuff is compacted and heated volcanic fly-ash, an thus a metamorphic rock. Tuff is easily worked; and, depending on the mineral content of the source volcanism and subsequent geologic processing, comes in a variety of colors and textures. Fine grained tuff takes a good polish. One form of tuff is Kimberlite, called “Blue Stone” in the South African diamond fields.
Peoples as diverse as the Romans and the Rapa Nui People of Easter Island have used tuff in statuary and as a building material. The Moai heads on Easter Island are carved out of a basaltic brown tuff (tholeitic tuff).
When hot lava encounters cold sea water, the result boiler explosion produces abundant amounts of volcanic ash which then accumulates as deposits of tuff, extending the base of the volcano out into the ocean and building up cinder cones. Diamond Head in Hawaii is a classic example.