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Glass Sold as Crystal: Opalite, Goldstone, K9 and Slag \"Obsidian\"

By Digital Towns Crystals ยท Last reviewed October 8, 2026

Short answer: several of the most common "crystals" in shops are factory glass. Opalite is usually milky glass or plastic, goldstone is glass grown with copper crystals, K9 "crystal" is optical glass, Andara and bright blue "obsidian" are colored glass or smelting slag, and much cheap moldavite is green bottle-type glass. None is a mineral. Glass is fine to buy when it is named as glass; the problem is a label that implies a mined stone. Bubbles, swirls, mold marks and impossible colors give most of it away.

What makes glass such an easy imitation?

Glass can be melted, colored with a pinch of metal oxide and poured or pressed into any shape, at a fraction of the cost of mining and cutting a stone. It takes a good polish and has a bright, vitreous luster. To a casual eye, a clear glass point and a quartz point look the same.

What glass cannot copy is the inside of a crystal. A mineral has an ordered atomic lattice, growth zoning, crystal faces and inclusions of other minerals. Glass is amorphous: its atoms are frozen in a disordered state, so it has no crystal faces of its own, no cleavage and no double refraction. Its typical inclusions are gas bubbles and flow swirls from the melt. Those differences drive every test below.

There is a catch. Some natural materials are glass too. Obsidian, moldavite and Libyan desert glass are all natural glasses, so "it is glass" is not the same as "it is fake". For those, the question is whether it is the natural glass or a manufactured one.

Which "crystals" are actually glass?

Opalite

Geology.com explains that "opalite" has two meanings: a natural common opal without play-of-color, defined in glossaries as early as 1945, and a marketing term for man-made opal-like glass and plastic, first used that way in the 1980s. The milky, blue-white tumble that glows orange against a dark background is the man-made version. The glow is an optical effect of the glass, not opal fire.

Goldstone

Goldstone is aventurine glass. According to Geology.com's goldstone article, copper oxide is dissolved in molten glass, and very slow cooling gives the copper ions time to find one another and grow into octahedral copper crystals; the slower the cooling, the larger the crystals. The sparkle is those tiny copper triangles, roughly 0.1 mm across, catching light at different depths. Blue goldstone gets its color from cobalt, green from chromium and purple from manganese. Modern production began in Murano, Italy, in the 17th century, and an earlier goldstone-like amulet from 12th to 13th century Iran is known. Its natural look-alike is sunstone feldspar, compared in sunstone vs goldstone.

K9 "crystal"

K9 is a Chinese grade name for a clear optical glass, used for engraved blocks, prisms, faceted pendants and the polished "crystal" balls and pyramids sold for display. It is bright and flawless because it is melted and annealed, not because it is exceptional quartz. Calling it "crystal" follows the glassware habit of the word, not mineralogy. The EPI gem laboratory in Germany notes that using "crystal" for non-crystallized products is misleading and is prohibited in European countries.

Slag and colored glass sold as obsidian

Natural obsidian is black, brown, gray or smoky, sometimes with sheen or rainbow layers. The EPI lab states that natural obsidian never occurs in transparent blue, red or yellow, and that a blue to green veined glass slag, a waste product of the iron smelting industry, has been sold with "astonishing persistence" as blue obsidian. GIA's Summer 1998 Lab Notes include a case of manufactured glass represented as "green obsidian". The International Gem Society likewise warns that artificial blue, green or red glass is passed off as obsidian in these rare colors.

Andara

"Andara crystal" is sold in aqua, green and other colors with stories of natural deposits. EPI's X-ray fluorescence tests found its chemistry matches artificial soda-lime glass, the composition of bottle glass: about 22% sodium oxide and 3% aluminum oxide, against roughly 6% and 10% for natural obsidian.

Helenite and "Mount St. Helens obsidian"

Helenite is a trade name for a man-made glass produced using volcanic ash from the 1980 eruption of Mount St. Helens. Geology.com notes it was not produced during the eruption, its color comes from colorants added to the melt, and its hardness is only 5 to 5.5. It is a souvenir product, not a natural obsidian.

Fake moldavite

Geology.com warns that much of the moldavite offered for sale, especially online, is green man-made glass whose origin is not disclosed, and that a faceted moldavite over two carats, or any moldavite without eye-visible bubbles, has a good chance of being fake. The full checklist is in the guide to fake moldavite.

How can you tell glass from a natural stone?

These checks work on loose pieces and need only a loupe, a light and patience.

  1. Look for round bubbles. Spherical or elongated gas bubbles are the signature of a melt. Crystals contain fluid inclusions, mineral needles, veils and growth zoning, but not floating round bubbles. Natural glasses have bubbles too, so this test separates glass from crystal, not natural glass from man-made.
  2. Look for swirls and flow lines. Curved streaks of slightly different color or refraction show where the melt was stirred or poured.
  3. Check the surface for molding. Rounded facet edges, slightly dished faces, seam lines and an identical shape across a whole tray point to pressed glass. Hand-cut stones vary.
  4. Ask whether the color is possible. Transparent blue, red or yellow "obsidian", glowing milky "opal" with no play-of-color, and neon green "moldavite" are colors natural material does not have.
  5. Compare against hardness. Quartz, at Mohs 7, scratches window glass. Most glass sits around 5 to 6, like obsidian at about 5.5. Test only on a hidden spot of a piece you own, and see the guide to real or fake tests for safe methods.
  6. Use polarized light if available. Glass is singly refractive and stays dark between crossed polarizers, apart from strain patterns. Quartz is doubly refractive and blinks light and dark as it turns.
  7. Distrust perfection at a low price. A flawless, water-clear 100 mm "quartz" sphere for the price of a dinner is almost certainly glass.

Separating natural obsidian from manufactured glass

Here the material is glass either way, so look at what is inside it. The IGS notes that natural obsidian often contains crystallites that stand out in high relief under magnification, along with straight banding and needles. It lists a refractive index of 1.48 to 1.51, usually 1.49, and a typical specific gravity of 2.33 to 2.42. Snowflake spots, sheen layers and mahogany streaks are formed in the volcano. Bright transparent color, perfect clarity and vivid streaky slag patterns are formed in a furnace.

What does the law say about naming glass?

The U.S. Federal Trade Commission's Jewelry Guides say it is unfair or deceptive to use a gemstone name, or words like "stone", "gem" or "birthstone", to describe an imitation unless the name is immediately preceded, with equal conspicuousness, by a word such as imitation or simulated. The guides add that "faux" is not an adequate disclosure. So "goldstone glass", "opalite (man-made glass)" and "glass sphere" are fine; "blue obsidian" for slag or "natural opalite" for glass are not.

Choosing natural glass in our Market

We do not sell opalite, goldstone, K9 or slag as stone. If you want glass that nature made, these are the real thing, and each shows features a furnace does not copy. The obsidian guide covers the volcanic material in depth.

  • Black Obsidian Sphere: polished volcanic glass. Look for the faint gray banding and tiny crystallites that appear under strong light, rather than perfect uniformity.
  • Mahogany Obsidian Tower: black glass with reddish brown iron oxide streaks, a pattern that follows lava flow rather than a mold.
  • Rainbow Obsidian: shows sheen bands only at certain angles, caused by aligned microscopic inclusions; dyed or coated glass shows color from every angle.
  • Libyan Desert Glass: a natural, pale yellow silica glass from the Sahara, with sand-frosted surfaces and natural bubbles and wisps.
  • Moldavite: natural impact glass from central Europe; the sculpted, etched surface and internal bubbles are what to check against any too-bright, too-smooth green glass.
  • Clear Quartz Pyramid: a real quartz pyramid to compare with glass versions. Natural quartz usually shows a few veils or inclusions; a flawless pyramid at a low price is the K9 tell.

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Frequently asked questions

Is opalite a real crystal?

Most opalite sold today is man-made glass or plastic, not a crystal or a mineral. The name once also meant natural common opal, but the milky blue-white tumbles that glow orange are manufactured. Buy it as decorative glass, priced accordingly.

Is goldstone a natural stone?

No. Goldstone is glass in which copper crystals grow during very slow cooling, giving the sparkle. It has been made since at least the 17th century in Italy. Blue and green goldstone are colored with cobalt and chromium.

What is K9 crystal?

K9 is a grade of clear optical glass. It is used for engraved awards, faceted pendants and polished spheres and pyramids. It is not quartz, and products made from it should be sold as glass rather than as natural crystal.

Is blue obsidian real?

Natural obsidian does not occur in transparent blue. Bright blue, aqua or green "obsidian" is manufactured glass or industrial slag. Natural obsidian is black, brown or gray, sometimes with a blue or rainbow sheen on a dark body.

Is glass a bad buy?

Not if it is labeled. Goldstone, art glass and optical glass can be beautiful and inexpensive. The problem is paying a mineral price for glass, or buying it believing it formed in the earth.

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