Amethyst is quartz coloured purple by iron and natural irradiation. It is abundant, inexpensive in commercial grades, and — because of that — imitated mostly at the low end by purple glass, while at the higher end it is synthesised hydrothermally in large quantities for the jewellery trade. The glass is easy to catch. The synthetic is not.
What amethyst is
Quartz (silicon dioxide). Properties: RI 1.544–1.553 (two readings), SG 2.65, hardness 7, doubly refractive, weakly pleochroic (purple / reddish purple), inert under UV. Colour ranges from pale lilac to deep purple; the deepest colour is usually concentrated in angular zones and at the tips of crystals.
Amethyst versus glass
Polariscope. Amethyst blinks four times per rotation; glass stays dark or shows patchy strain.
Refractometer. 1.544–1.553 with two readings versus a single glass reading around 1.5.
Microscope. Glass shows round bubbles and swirl marks. Amethyst shows angular colour zoning, planar "fingerprints", and sometimes the characteristic "tiger stripe" or zebra-stripe pattern from polysynthetic twinning — a feature strongly associated with natural amethyst.
Feel and weight. Quartz is cold; SG 2.65 versus glass at about 2.5. Hints only.
Any one of the first three tests settles it. Quartz vs glass: how to tell real crystal from imitation.
Amethyst versus synthetic amethyst
Synthetic amethyst is grown hydrothermally from a seed and then irradiated to develop the purple. It is quartz — same RI, SG, hardness, polariscope reaction. It has been produced commercially since the 1970s and is very common in calibrated commercial goods.
Separation rests on the microscope and on structural features:
Twinning. Natural amethyst almost always shows Brazil-law twinning, visible as the tiger-stripe pattern between crossed polarisers in the right orientation. Most synthetic amethyst is untwinned or shows twinning confined to a plane near the seed — its absence across the stone is a strong pointer to synthetic.
Colour zoning. Natural: angular, following crystal faces, often in the classic "chevron" or in a concentration at the tip. Synthetic: zoning parallel to the seed plate, planar.
Inclusions. Natural: fingerprints, negative crystals, goethite needles in some material. Synthetic: usually very clean; occasionally fine "bread-crumb" or cobweb inclusions; seed plate sometimes visible.
Infrared spectroscopy separates the two reliably where the microscope cannot, on the basis of water-related absorption features. It is the confirmation method in difficult cases.
A bench laboratory can usually reach a conclusion on the twinning and zoning; a very clean, untwinned stone may be reported as synthetic amethyst or undetermined — advanced testing required, and honesty about that is the mark of a careful report.
Treatments
Heating. Amethyst heated to around 400–500 °C turns yellow or orange — most citrine is heated amethyst — or green (prasiolite). This is a treatment of the resulting stone; amethyst as sold is not heated.
Irradiation. Pale amethyst can be darkened by irradiation. Uncommon in the trade because natural colour is cheap.
Dye. Very low-quality pale or crackled quartz is occasionally dyed purple; dye in fractures is visible under magnification.
Grading
Deep, evenly distributed purple with red flashes ("Siberian" in trade terms, regardless of source) is most valued; pale and unevenly zoned material grades lower. Opinion, not identity.
Frequently asked questions
Is synthetic amethyst fake?
It is quartz, chemically identical to natural amethyst. It is a fake only when sold as natural, which it commonly is in calibrated commercial jewellery.
Can I tell natural from synthetic at home?
Not reliably. Between two polarising filters, natural amethyst often shows a striped twinning pattern when viewed in the right direction; its absence is suggestive but not conclusive. A laboratory examination is needed.
Does amethyst fade?
Some amethyst fades with prolonged strong light or heat. Keep out of direct sun for long periods.
Why is deep amethyst more expensive than pale?
Rarity of saturated, evenly coloured material. The difference is grading, not identity — both are amethyst.
Is a geode amethyst natural?
Geodes and clusters are almost always natural (synthetic material is grown as slabs, not clusters). Dyed and heat-treated clusters exist; heated clusters turn orange.