Natural Glass Value, Price, and Jewelry Information

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Indochinite (656-755 ka, Pleistocene; Australasian Tektite Strewn Field, Guangdong, China) by James St. John, on Flickr

Natural Glass Value

The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.

Libyan Desert Glass is rare, Tektites are somewhat rare, Obsidian is common except in fancy colors.

Obsidian Highest values go to rare colors, the closer the stone is to transparent the higher the value. Faceted stones highest. Chatoyant somewhat higher. All others are close to the same value, with cutting costs greater than material cost.
Tektites Size, transparency, color, and quality of cutting.
Libyan Desert Glass Size, transparency, color, and quality of cutting.

Natural Glass Information

Data	Value
Name	Natural Glass
Is a Variety of	Glass
Varieties	Macusanite, Obsidian, Apache Tears, Fire Obsidian, Mahogany Obsidian, Rainbow Obsidian, Sheen Obsidian, Snowflake Obsidian
Stone Sizes	Tektites to ~25 carats, obsidian and Libyan Desert Glass often weigh several pounds
Heat Sensitivity	Yes
Formula	Primarily silicon
Colors	Obsidian: black, brown, gray, sometimes spotted or banded. Rarely, red, green, orange, blue, purple. Moldavite: yellowish to grayish green. Libyan desert glass: light yellow to greenish yellow.
Fracture	Conchoidal
Hardness	5-6
Cleavage	None
Wearability	Poor
Crystallography	Amorphous.
Refractive Index	Varies by type of natural glass, 1.46-1.69. See Gem Listings for specific varieties.
Birefringence	None
Luminescence	Not diagnostic.
Luminescence Present	No
UV Long	Not diagnostic.
UV Short	Not diagnostic.
Absorption Spectrum	Not diagnostic.
Pleochroism	None.
Luster	Vitreous.
Polish Luster	Vitreous.
Specific Gravity	2.20-3.00 (See chart below).
Enhancements	None.
Transparency	Transparent to Opaque
Phenomena	Iridescence and chatoyancy (Obsidian); others none.

Forms of Natural Glass used in Jewelry

Glass comes in several natural forms; all of these are all used in jewelry:

Obsidian

Obsidian is the most common natural glass. It is formed by volcanism and large pieces are common. It is frequently cut into cabochons and carved. The broken edges are sharper than any steel knife, so it is also used for scalpels, arrow heads, knives and scrapers.

Tektites

Tektites are created by meteorites striking the earth. The heat of the impact melts local sand, creating natural glass and scattering it over a wide area. The best known tektite is Moldavite, which is found in the CzechRepublic. Other tektites are found in several locations around the world.

Bubbles are common and numerous in tektites. It is very challenging to facet a stone and not have any breaking the surface. If a bubble reaches the surface on the pavilion, it has a minimal effect on value, based on how visible it is. On the crown it is conceded a significant blemish and greatly reduces value.

Libyan Desert Glass

The source of Libyan Desert Glass is a mystery. It is a transparent, light yellow glass that is usually faceted.

Mount Saint Helens

Mount Saint Helens glass is relatively new to the market. While there are reports of natural glass being found in the lava flows, that is not what is being sold. The ash is melted and coloring agents are added. This is not a natural product, although it does have a special romance that is not found with other glasses.

Natural glasses have the same bubbles and swirl lines of man made glass. (See Identifying Inclusions.) Its optical and physical properties are the same as untreated glass. However, most of our glass simulants have additives that raise their RI and SG above natural glass. Natural glass is also likely to have mineral inclusions that are not found in man made material. (See Identifying Characteristics.)

Properties

Obsidian

Source	Hardhigh	Hardlow	SGhigh	SGlow	RIhigh	RIlow	Birefringence	O	Dispersion
Natural	6	5	3.00	2.25	1.69	1.480	–	A	–

Moldavite

Source	Hardhigh	Hardlow	SGhigh	SGlow	RIhigh	RIlow	Birefringence	O	Dispersion
Natural	5.5	5	2.40	2.32	1.69	1.480	–	A	–

Libyan Desert Glass

Source	Hardhigh	Hardlow	SGhigh	SGlow	RIhigh	RIlow	Birefringence	O	Dispersion
Natural	5.5	5	~2.20	~2.20	~1.46	~1.46	–	A	–

Colors

Obsidian, black, brown, gray, sometimes spotted or banded. Rare, red, green, orange, blue, purple

Moldavite, yellowish to grayish green

Libyan Desert Glass, light yellow to greenish yellow

Identifying Characteristics

Obsidian gas bubbles frequently elongated, crystallites, (small nearly crystalline shaped mineral inclusions,) stubby needle like inclusions. May be banded or have closely oriented inclusions that cause sheen.

Moldavite gas bubbles, swirl lines, roiled effect

Libyan Desert Glass irregularly shaped gas bubbles, tiny rounded grains

Variety and Trade Names

Apache Tears Rounded pieces of obsidian, usually transparent to translucent and gray or brown.
Banded Obsidian has curved bands
Mahogany Obsidian has brown bands in black or gray base, resembling wood grain
Onyx Obsidian has straight banding
Rainbow Obsidian shows iridescent colors
Sheen Obsidian has a gold or silver sheen when viewed under proper lighting
Snowflake Obsidian had white or light gray inclusions resembling snowflakes, on dark gray or black base