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Which Crystals Are Heat Sensitive, and How Hot Is Too Hot?

By Digital Towns Crystals · Last reviewed October 8, 2026

Short answer: the crystals most likely to be hurt by heat are opal (crazing), fluorite and kunzite (cleavage cracks), amethyst, citrine and topaz (fractures from sudden temperature change, color loss at high heat), celestite and kunzite (fading), apophyllite (flaking as it loses water) and selenite (turning to anhydrite near 200 °C). In practice, sudden change does more damage than steady warmth: a stone moved from a hot car into cold water is at far greater risk than one left warm.

How hot does a crystal actually get in a car or a sunny window?

Hotter than the air. Meteorologist Jan Null, who measured vehicle heating for NoHeatStroke.org at San Jose State University, reports that on days in the 80s °F a dark dashboard or steering wheel commonly reaches 180 to 200 °F, about 82 to 93 °C, and that a black asphalt parking lot surface exceeds 160 °F. The vehicle heating page explains why: sunlight passes through the glass and heats the objects it strikes, and those objects then heat the trapped air.

A crystal sitting on a dashboard or a back shelf behaves like that dashboard. Dark stones and stones resting on dark surfaces heat fastest. The same FAQ notes that a trunk stays cooler than the cabin, about 5 to 10 degrees after an hour in his readings, because no sunlight reaches it. If stones must ride in a car on a summer day, the trunk, in an insulated bag, is the better place.

Most minerals survive 90 °C without any change at all. The danger comes from three things: steep temperature differences across the stone, minerals that change color at relatively low temperatures, and minerals that hold water in their structure.

Why does sudden temperature change crack crystals?

When a stone warms, the surface expands before the core does. If the difference is large enough, the strain opens the weakest planes in the crystal. Three features make a stone vulnerable:

  • Cleavage. A plane of weak bonding gives a crack an easy path. Fluorite has four directions of perfect cleavage, noted by Geology.com as unique among common minerals. Kunzite has two, and topaz has one.
  • Inclusions. Fluid inclusions expand much more than the host crystal, and solid inclusions expand at a different rate. Both can crack a stone from inside.
  • Existing fractures. A partly closed old fracture or a cleavage crack already started is the first thing to open.

This is why the gem references keep repeating the same warning. GIA's amethyst care guide says abrupt temperature changes can cause amethyst to fracture, and GIA uses near-identical wording for citrine, kunzite, topaz and moonstone. The IGS gemstone care guide describes opal as very sensitive to thermal shock, hot or cold, which causes crazing.

The heat table

Mineral Mohs What heat does Known temperatures
Opal 5 to 6.5 Crazing from heat or sudden change; heat from intense light can do it Avoid any rapid change
Fluorite 4 Cracks along four cleavages; some colors phosphoresce or change Red and green fluorite phosphoresce above 100 °C
Amethyst 7 Fractures from abrupt change; color shifts with strong heat Turns brown, red or green at 400 to 500 °C
Citrine (heated) 7 Fractures from abrupt change; may fade with heat No steam cleaning
Kunzite 6.5 to 7 Breaks from sudden change; color fades with heat Avoid sunny dashboards entirely
Topaz, yellow to brown 8 Breaks along cleavage; brown colors can fade Keep from high heat
Celestite 3 to 3.5 Blue fades Fading reported from torch heat over 200 °C
Apophyllite 4.5 to 5 Flakes apart as it loses water Jeweler's torch heat
Gypsum (selenite) 2 Loses water, turns to anhydrite About 200 °C
Malachite 3.5 to 4 Rated very heat sensitive Avoid hot water
Turquoise 5 to 6 Discoloration and surface damage High heat
Maxixe beryl 7.5 to 8 Deep blue bleaches Above 100 °C

Why do opals craze, and can heat cause it?

Opal is amorphous silica that holds water in its structure. As it loses water or meets a sudden temperature change, the surface can develop a web of fine cracks called crazing. GIA's opal care guide says opal is generally stable, but heat from intense light can cause crazing, and high heat or sudden temperature changes can also fracture it. That makes a display spotlight a real risk for opal, not only a hot car. Ethiopian Welo opal is often hydrophane and absorbs water readily, so it should also be kept away from hot, wet cleaning. The opal guide covers types and localities.

Is fluorite really that fragile with heat?

Yes, but for mechanical reasons more than chemical ones. Fluorite is the Mohs 4 reference mineral and breaks easily into octahedra along its four cleavage directions, so a thermal gradient finds a ready-made path. The IGS fluorite entry calls it too fragile for most jewelry use because of perfect cleavage, brittleness and low hardness.

Heat also changes fluorite in less obvious ways. Minerals.net notes that red and green fluorite become phosphorescent if heated above 212 °F (100 °C), that the chlorophane variety glows green when heated, and that color caused by hydrocarbons can be removed by heating. A fluorite on a hot LED base or a sunny sill is not going to melt, but its color and its integrity are both at stake over time. More on the species is in the fluorite guide.

Does heat change the color of amethyst and citrine?

At kitchen temperatures, no. At the temperatures used in treatment, yes. The IGS amethyst entry states that when heated to 400 to 500 °C, amethyst may turn brown, red and sometimes green; that green material is sold as prasiolite. Those are kiln temperatures, not car temperatures. The practical risks at everyday temperatures are fracturing from sudden changes and slow fading of already heat-treated citrine, which the IGS citrine entry says may fade when exposed to heat. Light fading is a separate subject covered in crystals that fade in the sun.

Which minerals change chemically when heated?

  • Apophyllite. Its name means "to leaf off." Minerals.net's apophyllite entry explains that it exfoliates, flaking apart like a leaf, when heated because it loses water from its structure. The IGS apophyllite entry warns that even a jeweler's torch can make it lose flakes.
  • Gypsum (selenite, satin spar). Geology.com notes that gypsum heated to about 200 °C gives up its water and converts to anhydrite. A selenite tealight holder is fine with a cool LED candle, but not with a real flame burning down into it; see storing selenite.
  • Celestite. The IGS celestite entry says jeweler's torches can easily exceed 200 °C and cause its colors to fade.
  • Turquoise. GIA's turquoise guide says high heat can cause discoloration and surface damage.

How should heat-sensitive crystals be handled day to day?

  1. Let stones equalize. After a cold car ride or a hot one, leave a stone in its packaging for an hour before unwrapping or washing it.
  2. Wash only in lukewarm water. Never run hot water over a cold stone or drop a warm stone into cold water.
  3. Skip steam and ultrasonic cleaners for opal, fluorite, kunzite, topaz and anything with visible inclusions.
  4. Watch display lighting. Old halogen spotlights and small, close bulbs heat the surface beneath them. Keep opal and kunzite out of hot light, and feel the stone after an hour under a light: if it feels warm, move the lamp.
  5. Never leave stones on a dashboard or rear shelf. If they must travel in summer, use the trunk and an insulated bag.
  6. Keep candles and selenite apart unless the candle is a flameless LED.
  7. Ship in winter with insulation so a parcel left on a cold porch does not meet a warm room too abruptly.

Choosing heat-sensitive crystals from our stock

When you buy, look closely at listing photos for existing cleavage cracks in fluorite and fine crazing lines in opal: a stone that already has them will open further with the next temperature swing. Ask whether amethyst, citrine or topaz has been heat-treated, and treat heated stones as more prone to fading.

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Frequently asked questions

Can crystals be left in a hot car?

Most hard stones survive, but it is a poor habit. A dark dashboard can reach 180 to 200 °F on an 80s °F day, and heat-sensitive stones such as opal, kunzite and fluorite can crack or fade. The trunk, in an insulated bag, stays cooler.

Can crystals crack from heat?

Yes, mostly from sudden change rather than steady warmth. GIA warns that abrupt temperature changes can fracture amethyst, citrine, kunzite, topaz and moonstone. Stones with cleavage or fluid inclusions, such as fluorite and opal, are the most vulnerable.

At what temperature does amethyst change color?

The International Gem Society reports that amethyst may turn brown, red or green when heated to 400 to 500 °C. Those are treatment temperatures, far above anything a home or car reaches, so everyday risk is fracturing rather than color change.

Is it safe to put a candle in a selenite holder?

Use a flameless LED candle. Gypsum gives up its water and converts to anhydrite at about 200 °C, and a flame burning down into a satin spar holder can scorch and crumble it. The holder itself is fine at room temperature.

Why did my opal crack?

Most often from drying or a sudden temperature change. GIA notes that heat from intense light can cause crazing in opal and that high heat or rapid changes can fracture it. Keep opal away from spotlights, sunny windows and hot water.

Sources