My mineral I had to do research on
was Greywacke. It is most likely to be found in Sedimentary Rocks. It's
color's are various shades of dark grey and greenish grey. When it is used
on a streak pad it's streak is invisable. Some of it's varities are it's
feldspatic Greywacke, which is very rich in feldspar.
Some of it's distinctive features
are that it is a poorly sorted dark grey to greenish sandstone.
It's occurrence is world wide but
especially bordering ancient fold and mountain ranges.
Some practical uses for greywacke
is that it is used as a building stone.
Some of the composition's of greywacke
are Quartz, Plagioclase and tiny rock fragments set in a matrix of microscopic
Feldspar, Quartz, Clay and other minerals that are too small to determine
without a microscope.
Here are some links to a good rocks and minerals sites: