Oligoclase: A
Soda Lime Feldspar. It occurs usually massive, and occasionally in
crystals. Color white, grayish, grayish-white, reddish-white, greenish.
Transparent and subtranslucent. It is found in Stockholm, Finland,
Bohemia, Ireland, in pumice in Peru and in Obsidian in Mexico, besides
numerous localities in the United States.
Olivenite: An
Arsenate of Copper. Arsenc 40%, Cupric Oxide 56%. This mineral occurs
in prismatic crystals, and in granular and fibrous crusts, olive-green
in color, passing into leek and pistachio-green; also wood-brown and
grayish-white. Subtransparent to opaque. The crystallized varieties
occur in Cornwall, Redruth and Devonshire and Cumberland. In the
United States in Utah, in the Tintic District, it occurs both as
crystals and as "wood-copper."
Olivine: A
variety of Chrysolite, dark yellowish-green to olive-green in color. A
variety characterized chemically by the presence of titanium, deep
yellow or red with strong
pleochroism, is called Titan-Olivine. Olivine occurs in the lavas of
many volcanoes, in the Sandwich Islands; near Naples, and in rocks
in Vermont and New Hampshire.
The
only member of the group that is of any economic importance is a pale
yellowish-green transparent olivine, which is used in jewelry under the
name of Peridot. Gem material is found in Arizona, scattered loose in the soil.
Onyx: This
is a very evenly banded Agate or Chalcedony in which there is marked
contrasts in colors. It was highly valued by the ancients and is much
used for cameos, the figure being cut in one layer, and the background
consisting of another layer, generally in another color. It is similar
to the oriental Alabaster used in Bible times to make ointment boxes.
Opal: Amorphous
Hydrous Silicate. A mineral characterized by its iridescent reflection
of light. The play of color in gem material is due to the interference
of light-rays reflected from the sides of thin layers of opal material
with different densities. The beauty of the Opal is due entirely to
this prismatic play of colors —the thin films break up and refract the
light in much the same way that a soap-bubble film reacts on light. The
value of an Opal largely depends on the brilliancy and richness of
these refracted colors, blues, purples, greens and reds being among
those especially prized.
It
occurs in crystals, massive, in stalactitic or globular masses, and in
an earthy condition. The precious Opal is a transparent
