The birthstones
for October are the opal or the tourmaline. 
Opal, admired for its distinctive play of color, is a semitransparent
solidified mineral composed of silicon and water. This unique
stone gets its name from the Latin opalus and Sanskrit
upala (precious stone). The Greek term opallios translates
"to see a change of color." There are four main types
of opal (white, black, water, and fire). avalable in a variety
of colors and patterns.
Opal has been revered by many for centuries. The oldest known
opal mine is in Czechoslovakia and dates back to the 14th century.
Shakespheare praised opal as "the queen of gems" in
Twelfth Night. During the 19th century, the popularity
of opal suffered due in part to Sir Walter Scott's novel Anne
of Geierstein, in which opal was associated with bad luck.
In the late 1800s, Queen Victoria promoted Australian opals by
presenting opal jewelry to her children. To-day, Australia continues
to be a major opal source, supplying 85% of all the world's opals.
Mohs hardness: 5.5-6.5
Refractive Index: 1.44-1.46
Specific Gravity: 2 to 2.2
The toughness of opals is poor. As they are very delicate, clean
opals only with an untreated cloth.
Bolder opal is a clear water opal with brilliant color play and
a distinctive dark chocolate brown ironstone matrix. This intriguing
gemstone formed when silica bearing steam was forced into cracks
and crevices in ironstone boulders, then mineralized. Although
no two opals are identical, matched boulder opals are selected
for their similar color, matrix, and cut
Color: Naturally ranging in shades of pinkish white to deep royal
blue, these opals display an interesting mix of color.
Mexican fire opal is aptly named for its brilliant red to orange
body color. The color is due in part to the presence of iron oxide.
Mined in Mexico, this type of opal is more lightweight than other
opals, and it contains more water. Mexican fire opals tend to
have one body color and do not usually contain the flashes of
light and color typical of other opals. Available in translucent
and transparent varieties, Mexican fire opal is the only natural
opal than can be successfully faceted.
The Aztec Indians of Mexico were among the first to know about
this fiery gem. Europeans learned about Mexican fire opals when
the Spanish conquistadors returned from the New World with this
brilliant gem.
The basic factors that influence an opal's value are size, cut,
rarity, base color, fire color (colors produced when light is
diffracted out of the stone), fire intensity, fire pattern (the
pattern made by the play of color), and fire consistency.
Tourmaline
has been used as gem material for over 2000 years. The Chinese
have long valued this beautiful gem; they've carved tourmaline
ornaments for headdresses, girdles, badges, and buttons. In 1703,
a package of tourmaline labeled "turmali" from Sri Lanka
was sent by mistake to a stone dealer in Amsterdam. (The name
tourmaline is said to have originated from this Sinhalese
term). The Dutch noticed that the stones attracted ashes and straw
when heated, and named the stones aschenstrekkers (ash
drawers).
Mohs Hardness: 7-7.5
Refractive Index: 1.624-1.644
Specific Gravity: 3.01-3.06
There are three gem varieties of tourmaline:
Pink to red tourmaline is known as rubellite or elbaite. Elbaite
is named after the Isle of Elba, the first known source of this
stone. The pink color is due to the presence of manganese. The
redder rubellite appears to be similar to a ruby. The rubellite
is found in Siberia, Burma, Sri Lanka, Brazil, the United States
of America (California), and Madagascar.
The blue variety of tourmaline is known as indicolite. Indicolite
is found in Brazil, United States (Colorado, Massachusetts, and
California), Namibia, Madagascar, and the Urals.
The green variety of tourmaline has no special name; therefore,
it is known as green tourmaline. Green tourmaline is found in
Brazil, the United States (Maine), Tanzania, Mozambique, Namibia,
the Urals, and Sri Lanka.
The toughness of tourmalines is good. Clean them only with jewelry
cleaner, soap, or an untreated cloth.
