The stone type obsidian

What is obsidian?

Obsidian forms when silica-rich lava cools so quickly that hardly any crystals have time to grow. The result is a glass-like stone with a smooth fracture surface and often sharp edges. Black obsidian is the best known, but there are also varieties with gold sheen, silver sheen, snowflake patterns or rainbow-like effects.

Because of its sharp fracture surfaces, obsidian was already used in ancient times for cutting tools, arrowheads, knives and decorative objects. For modern hobbyists, obsidian is mainly interesting as a collector’s stone, ornamental stone and sometimes as material for the rock tumbler or polishing barrel.

Is obsidian suitable for a rock tumbler?

Obsidian can be polished in a rock tumbler, but it is not a perfect beginner’s stone. The stone can have sharp edges, break glass-like and pick up small chips or pits during tumbling. This means obsidian needs more cushioning, careful sorting and monitoring than harder stones such as agate and jasper.

If you want to tumble obsidian, do not simply mix it with much harder stones. Preferably use a batch of obsidian or stones of a similar hardness, and make sure there is enough filler. Ceramic media or other suitable filler can help reduce hard impacts in the barrel.

• Use compact pieces: avoid extremely thin, splintery or already broken pieces.
• Watch out for sharp edges: obsidian can break in a glass-like and sharp way.
• Use enough filler: too much impact can cause chips and pits.
• Do not simply mix with hard stones: agate and jasper can damage obsidian.
• Check regularly: it is better to stop in time than to wear the stone down too far.

Important: obsidian can polish beautifully, but the result is less predictable than with agate. For a first batch, hard, compact stones are often easier.

Polishing obsidian: what should you pay attention to?

Obsidian has a different structure from, for example, quartz or agate. Because of its glass-like nature, the stone can form sharp splinters and become damaged more quickly when the batch is too empty or when there are too many hard stones in the same barrel.

In practice: want to get started easily? Then also view the category rough stones for rock tumblers and first choose harder, more compact beginner stones.

Obsidian as a polished drop or pendant

Obsidian is often used for pendants, cabochons, beads and decorative shapes. A smoothly polished obsidian drop clearly shows why the stone is so popular: the lustre is deep, dark and almost mirror-like.

Beautifully polished, but vulnerable in rough tumbling

A polished obsidian pendant or drop is usually already shaped and finished. Such pieces should not simply be put back into a rock tumbler. The tumbler is mainly intended for rough stones that still need to be rounded and polished.

If you want to work with obsidian yourself, it is better to start with rough pieces intended for that purpose and use enough cushioning in the barrel.

Polishing obsidian in stages

Obsidian requires a careful approach. A standard programme for hard stones can be too aggressive, especially if the pieces are thin, sharp or splintery. The steps below are intended as a general guide. For the full process, also see the stone polishing step-by-step guide.

Note: obsidian can achieve a deep shine, but chips and small pits are easier to see when tumbling is done incorrectly than with some other stones.

Different types of obsidian

Obsidian is often named after its colour, lustre or pattern. Some names are geological varieties, while others are mainly commercial or descriptive names. For the rock tumbler, the structure is more important than the name.

Common mistakes when polishing obsidian

Obsidian can look strong and hard, but its glass-like nature makes the stone more prone to sharp fractures. In particular, a barrel that is too empty, impacts that are too hard and mixing it incorrectly with other stones can cause problems.

• Using too little filler: the stones then collide more strongly and can chip.
• Mixing with much harder stones: agate and jasper can damage obsidian.
• Using splintery pieces: thin edges can break off quickly.
• Starting too aggressively: coarse grit can remove too much material.
• Not cleaning between stages: leftover coarse grit can create new scratches.
• Expecting too much from fragile pieces: not every obsidian stone becomes perfectly even.

Does obsidian occur in the Netherlands?

Obsidian forms in volcanic areas. The Netherlands has no active volcanoes and obsidian is not a normal natural find here for hobby collectors. Obsidian bought in the Netherlands is almost always imported from countries with volcanic sources.

Known source areas for obsidian include Mexico, Turkey, the United States, Armenia, Italy, Iceland and other volcanic regions. If you want to search for stones yourself in the Netherlands and Belgium, also read which stones you can find in the Netherlands and Belgium.

Buy obsidian or keep it as a collector’s stone?

Beautiful obsidian with rainbow sheen, gold sheen or silver sheen can be attractive as a specimen, pendant or cabochon. Such pieces are not always ideal to put into a tumbler batch. For tumbling, it is better to use suitable rough obsidian or a mix that has been specifically intended for the polishing barrel.