October is one of the few months with two official birthstones. Opal is the traditional one. Tourmaline was added to the modern list in 1952, mostly as a sturdier, more affordable alternative. They could hardly be more different: one is a hardened silica gel with no crystal structure at all, the other is one of the most chemically complicated crystals in the mineral world.
Here is what is actually going on inside each stone, how to tell the genuine article from the imitations, and a few pieces from the shop if you are shopping for an October birthday.
Opal: a stone made of tiny spheres
Opal is hydrated silica, the same basic ingredient as quartz with water locked inside. Unlike quartz, it never organizes into crystals. Under an electron microscope, precious opal turns out to be built from microscopic silica spheres, each a few hundred nanometres across, stacked in tidy rows.
That stacking is the whole trick. When the spheres are uniform and evenly packed, they diffract light the way the grooves on a CD do, splitting white light into flashes of colour. Gemologists call it play-of-colour. Change the angle and you change which colours you see, which is why a good opal seems to switch on and off as it moves. Where the spheres are jumbled or uneven, there is no diffraction, and you get common opal: still opal, still attractive, just without the flash.
Welo opal from Ethiopia
Most of the opal on the market today comes from Australia or from the Welo (Wollo) region of Ethiopia, where large deposits were found in 2008. Welo opal tends to have a pale, milky to transparent body with bright green, orange and pink play-of-colour.
One thing every Welo owner should know: much of it is hydrophane, meaning it can absorb water. Soaked, it may turn temporarily clear or lose its colour, then recover as it dries out. Keep it away from long soaks, lotions and harsh cleaners, and wipe it with a dry soft cloth.
Our Ethiopian Welo opal bead necklace is a good example: graduated beads, each showing its own flash pattern, on a sterling silver clasp. Because play-of-colour depends on how each bead was cut relative to its internal structure, no two beads on the strand show exactly the same colours.
Common opal
Opal without play-of-colour still comes in beautiful body colours, including pink, blue and green. Our pink opal heart carving is common opal: soft, even pink, polished smooth, and a far gentler price point for someone who wants an October stone as a desk or shelf piece rather than jewelry.
How to spot fake or altered opal
- Glass and resin imitations often show colour in flat patches with a repeating, almost scaly pattern. Natural play-of-colour is irregular and shifts with depth.
- Lab-grown opal is real opal made in a lab. It is often recognisable by a columnar "snakeskin" pattern when viewed from the side. It is not a fake, but it should always be disclosed.
- Doublets and triplets are thin slices of opal glued to a backing and sometimes topped with a clear dome. Look at the edge of the stone: a straight glue line is the giveaway.
- Dyed or smoke-treated opal is sold to imitate rare black opal. Colour that sits only on the surface, or a price that seems too good, is a reason to ask questions.
Tourmaline: the striped crystal
Tourmaline is a borosilicate, and it is a family rather than a single mineral. Swap in different elements (iron, lithium, manganese, magnesium) and you get different species and colours: pink, green, blue, watermelon, and black. The black variety, called schorl, is iron-rich and by far the most common tourmaline on Earth.
Tourmaline grows as long prisms with a rounded triangular cross-section and fine lengthwise grooves called striations. Those grooves are one of the easiest ways to recognise it in the rough. It is also hard, around 7 to 7.5 on the Mohs scale, which makes it more forgiving for everyday wear than opal at roughly 5.5 to 6.5.
Tourmaline has a genuinely odd physical property: it is pyroelectric and piezoelectric. Heat it or squeeze it and it develops a small electrical charge at opposite ends of the crystal. In the 1700s Dutch traders noticed warm tourmaline from Sri Lanka pulling ash out of their pipes. That is real, measurable physics, though the charge is tiny and fades quickly.
How to tell real black tourmaline
- Look for striations. Raw schorl has parallel grooves running the length of the crystal. Moulded glass or resin does not.
- Check the fracture. Broken tourmaline shows an uneven to shell-shaped (conchoidal) break with a glassy lustre, and it is brittle across the crystal.
- Weight and temperature. Real stone feels cool and dense in the hand. Plastic warms quickly and feels light.
- Beware of dyed black stones passed off as tourmaline. A colour that rubs off on a damp cloth is not natural schorl.
Our best-selling piece right now is a raw black tourmaline pendant: a natural, unpolished schorl crystal in a silver-plated wire wrap. Because it is left rough, you can see the striations for yourself. Pair it with an adjustable black cord and it is ready to wear.
A note on birthstone meanings
You will find plenty of claims online about what opal or tourmaline "does". We treat those as folklore, not fact. Opal has a long and contradictory history, prized in ancient Rome and later considered unlucky after a 19th-century novel. If the cultural history interests you, our guide to gemstone folklore and the geology behind it covers it properly, with the science alongside.
October birthday gift ideas
- Under $15: the raw black tourmaline pendant.
- Around $20: the pink opal heart, for someone who would rather display a stone than wear it.
- A real statement: the Welo opal bead necklace. We have one.
For more ideas across every budget, browse our necklaces or see what has just arrived in New.
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