Hyalite Opal is of a hydrous silicon dioxide chemical composition.  It occurs in an amorphous crystal structure.  It comes colorless with a strong sheen in white, creme, yellow, green and orange colors. 

It can also be found in vibrant green and yellows, with the remarkable color-change and fluorescence phenomena. These colors, however, are very rare.

Hyalite has a hardness ranging from 5.5 to 6.5 on the Mohs scale. It is transparent to translucent with a glass-like and sub-vitreous luster.  Some Hyalite Opal gemstone shows a brilliant green fluorescence under UV light due to uranium impurities.  The fluorescence or luminescence in Hyalite Opals can also be seen in daylight conditions which is a spectacular sight to behold.  Though Hyalite Opal gemstones contain a small amount of radioactivity, it does not however pose any health danger to the wearer.

Hyalite Opal is the most unusual among the varied family of opals. 

Opal jewelry making goes as far back as 6000 years. 

Hyalite was named in 1794 by German geologist Abraham Gottlob Werner (1749-1817) from the Greek word (HÝALOS), meaning glass, for its resemblance to glass.

Opal means Precious Stone both in Greek [Opallios], Latin [Apalus] and Sanskrit [Upala].

There are two types of Opals:

“Precious Opal” and “Potch Opal” which consist of aggregated spheres of amorphous silica, with water filling the gaps in between.  The difference between the two is in the regularity of the sizes of the spheres and packing.

"Precious Opal" shows a play of colours ("opalescence") in white light that is due to diffraction from the regular packing of silica spheres of roughly equal size.

"Potch opal" includes the massive varieties that shows no play of colours.

Hyalite Opal, therefore, pertain to the precious Opals due to its play-of-colors.

Some Hyalite Opals have minute inclusions or trace elements of Uranyl (a residue of Uranium) which cause a fluorescent green glow under certain light conditions. Hyalite Opals that are colorless or very pale yellow can come alive with this electric otherworldly green glowing color in direct sunlight or ultraviolet light.

Hyalite Opal is also known as Opal-AN. Opal-AN is a term for amorphous opal with a glass-like structure. The "A" in the name stands for amorphous; the subscript "N" is to imply its structure is network-like similar to silica glass.  However, it still contains about 3-8% water, both as molecular water and bound in silanole (SiOH) groups.

This form of opal occurs as hyalite in volcanic and pegmatite environments where the silica deposits from the gas phase and forms globular and irregular crusts.  [mindat.org: Opal-AN]

However, if the packing of the spheres is similar to the structure of a gel, the subscript G ("gel-like") is added.  Thus, the term Opal-AG. 

Clear and colorless Hyalite Opals are often referred to as:

Muller’s Glass
Glass Opal
Water Opal
Grape Opal
Opal-AN – official name

When Hyalite Opals displays the fluorescent phenomenon it is often named:

Hyalite Opal
Electric Opal
Ice Opal

Usually, Hyalite Opals that are fluorescent or luminescent and are large and transparent enough are faceted.  However, Hyalite Opals are not larger than a carat or two with the majority being a fraction of a carat.  Not only that, 60-80% of the rough material is wasted when faceted.

Thus, because of this extreme rarity of finding a large and transparent Hyalite Opal, it is the translucent samples with gorgeous inclusions that are the most sought after.

Hyalite Opals have the same chemical composition as all other Opals, with the exception of some tiny inclusions or trace elements of Uranyl (a residue of Uranium) which causes its fluorescent green glow.

Opals are associated with passion and desires, lust and seduction, and hippie type of loving freely with no constraints.

Opals give rise to artistic creativity, originality and inspiration.

It can help the wearer discover truths by using his or her intuition, and helps him or her open up the heart to the people near the wearer.

Hyalite Opals are associated with the Crown Chakra, which concerns wisdom and knowledge. 

The green fluorescence of Hyalite Opals helps unblock the solar plexus chakra. Hyalite Opals as a whole help unblock all the Chakras.

Another name for Hyalite Opal is Waterstone as it contains a high-water content.  Thus, Hyalite Opals have a great beneficial influence over the body’s water content.  It can prevent both water retention and dehydration.  It also benefits the eyes, skin and hair.

Hyalite Opals are extremely rare so any well cut, transparent ones that exhibits the magical color-change phenomenon demand a high price.  For the most part, Hyalite Opals come in small sizes, and thus, anything that is over a carat or two will be much priced.

Hyalite Opals are not treated in any way to enhance its color.  

Opals have the same hardness as glass at 5.5 to 6.5 in Mohs hardness.  Thus, any ring using Hyalite Opal must come with a protective setting.  Since Hyalite Opals do not come in large sizes, it is rarely used in pendants, necklaces and brooch jewelry.  Hyalite Opals as side stones look stunning both in earrings and rings.

To clean your Hyalite Opal you only need a soft dry clean cloth.  Avoid using soap and water.

How to know that you have a real Hyalite Opal?  Shine a UV light on the gemstone.  A real Hyalite Opal turns green under UV light.  Sunlight will also do the trick.  Even on a cloudy day, Hyalite Opals still glow a ghostly deep green color.  This color change is known as fluorescence. 

Hyalite Opals can be found in Hungary, the Czech Republic and Italy (where it was known as Amiatite). The colorful gemstones can be found in Namibia, Madagascar and in the US; nonetheless, it is Mexico where the most spectacular Hyalite specimens have been found.