Friday October 13, 2017 9:30 AM -
Weekly on Monday, Tuesday, Wednesday, Thursday, Friday, and Saturday
Unit 1 Red Lion Yard, High Street Colchester, Essex, CO1 1DX
Dichroic glass is glass containing multiple micro-layers of
metal oxides which give the glass dichroic optical properties. The invention of
dichroic glass is often erroneously attributed to NASA and its contractors, who
developed it for use in dichroic filter. Dichroic glass dates back to at least
the 4th century AD as seen in the Lycurgus cup.
Multiple ultra-thin layers of different metals (gold,
silver), metal oxides (titanium, chromium, aluminium, zirconium, magnesium) and
silica are vaporised by an electron beam in a vacuum chamber. The vapour then
condenses on the surface of the glass in the form of a crystal structure. This
is sometimes followed by a protective layer of quartz crystal. The finished
glass can have as many as 30 to 50 layers of these materials yet the thickness
of the total coating is approximately 30 to 35 millionths of an inch (about 760
to 890 nm). The coating that is created is very similar to a gemstone and, by
careful control of thickness, different colours are obtained.
The history of the emerald is fascinating.. The name emerald
is a derivative of an ancient Persian word, coming to us through the corruption
of the Latin Smaragdus which really just means 'green gemstone'.
The first emerald mines recorded in history are the famous
Cleopatra mines rediscovered in 1818 in Northern Egypt. Cleopatra adored
jewellery and she loved the green gem stone above all others and once gave an
emerald with her portrait engraved on it to at least one favoured ambassador.
These ancient mines were exhausted long ago.
The emerald mines in Colombia have a history somewhat
similar to those in Egypt. When the Spaniards conquered the Incas, they saw
emeralds in their possession, but even under torture, these never revealed
their source. Years later, one mine in the Chivor area in Colombia was
discovered quite by accident
Pliny wrote, ?nothing is more intense than the green of
emerald? and ?sight is refreshed and restored by gazing upon this stone?.
Following his advise, Roman emperor Nero wore emerald sunglasses to watch the
The Moguls of India loved emeralds so much that they
inscribed them with sacred texts and wore them as talismans. This led to the
flourishing Jaipur cutting industry.
Emerald is said to give a supernatural ability to foretell
A surprising variety of virtues have been ascribed to
emerald. Among these, emerald was thought to improve its owner?s memory and
eloquence, and was also said to quicken intelligence. In a particular instance
of emerald?s use, as a measure against ills, women wearing the stone were
believed to be immune from epilepsy.
Fissures and fractures that are characteristic of emerald
are traditionally filled with oil to minimize their visual impact. Cleaning
emerald with an ultrasonic cleaner can remove or damage the oil, thus making
the fissures more visible. This is the same for most detergents as well so if
you have an emerald ring remove or cover before washing.
In the early 1990s a new type of opal surfaced in Ethiopia
which was called Chocolate opal because of the chocolate colour inside the
nodule. These opals are found in a round nodular form with in a 3 meter thick
layer of welded volcanic ash. Only about 1% of these nodules contain colour.
The colours are very striking with red being common and blue quite rare which
is the opposite to Australian opals. It has some magnificent patterns and
brilliant colours and is called Ethiopian fire opal.
Later opal was found in Gondar which was at first called desert
opal but it is from a plateau in the highlands. The main field which is
creating a lot of excitement now is from a field called Welo.
This is found in a plateau 2500 to 3299 meters. Only the
locals are allowed to mine this field and the government has even supplied
basic tools. They work the horizontal level of these steep mountains and
unfortunately there have been fatalities due to the rock top collapsing. This
field produces a variety of crystals, brown base and even black material. A
very small percentage is called gum opal as if you wet your hand the opal
sticks to it like gum!
Opals from this field are known as Ethiopian opal from
A large majority of this material is hydrophane as if it is
soaked in water the base colour can become clear increasing the play of colour
or it can sometimes vanish. When dry this material is very bright. If it gets
wet it may take a few weeks to dry out but don?t hurry the process. This is why
some cutters cut it dry to prevent this.
Good Ethiopian opals have diverse play of colours from Neon
reds, oranges, green, blue, white, yellow, brown and contra luz fire. Ethiopian
fire opals are popular as they have striking pattern formations which make each
opal so unique
Ethiopian Opals, from North Africa, have only begun to be
mined recently. However, anthropologists report that around 4,000 years BC,
early man used opals to make tools, which means that Africa mined opals before