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Crystal Sizes and Weights Decoded: Units, Conversions and a Sphere Weight Chart

By Digital Towns Crystals · Last reviewed October 8, 2026

Short answer: a crystal listing should give a size in millimeters or inches and a weight in grams, pounds or carats. One inch is 25.4 mm, one pound is 453.6 g, one ounce is 28.35 g, and one metric carat is exactly 0.2 g, so a gram is 5 carats. For a sphere, weight equals 4/3 × π × radius³ × specific gravity, which lets you check any listed diameter against its weight in a minute.

Which units do crystal listings use, and how do they convert?

Listings mix three systems: metric (millimeters, centimeters, grams, kilograms), US customary (inches, ounces, pounds) and the jeweler's carat. The exact factors are fixed by definition, and the NIST Guide to the SI lists them.

Unit Equals Notes
1 inch 25.4 mm (2.54 cm) exact
1 ounce (avoirdupois) 28.35 g the everyday ounce
1 pound (avoirdupois) 453.59 g 16 ounces
1 troy ounce 31.10 g precious metals, not crystals
1 metric carat 0.2 g (200 mg) 5 carats per gram
1 kilogram 2.205 lb

The carat deserves a note. GIA defines a metric carat as 200 milligrams, divided into 100 points, and stresses that carat measures weight, not size. Carats make sense for cut gems; on a 700 g sphere they produce a number like 3,560 carats that sounds impressive and says nothing extra. When a mineral specimen is priced in carats, convert to grams before comparing it with anything else.

Watch for the troy ounce, too. It is about 10 percent heavier than the ordinary ounce and belongs to gold and silver; a crystal seller quoting "oz" almost always means the avoirdupois ounce.

How do you work out a sphere's weight from its diameter?

A sphere's volume is 4/3 × π × r³, the formula Wolfram MathWorld credits Archimedes with first deriving. Specific gravity (SG) is the density of a material relative to water, so in metric units it converts volume straight to weight: one cubic centimeter of a stone with SG 2.65 weighs 2.65 g.

The method:

  1. Halve the diameter to get the radius, and work in centimeters.
  2. Cube the radius and multiply by 4.18879 (that is, 4/3 × π) to get the volume in cm³.
  3. Multiply by the stone's specific gravity to get grams.

Specific gravity of common sphere stones

Stone SG used Source
Quartz (clear, rose, smoky, amethyst) 2.65 webmineral
Labradorite 2.69 (range 2.68 to 2.71) webmineral
Calcite 2.71 webmineral
Fluorite 3.18 webmineral
Gypsum (selenite) 2.31 webmineral
Pyrite 5.01 webmineral

Sphere weight chart

Diameter Volume Quartz 2.65 Calcite 2.71 Fluorite 3.18 Selenite 2.31 Pyrite 5.01
40 mm 33.5 cm³ 89 g 91 g 107 g 77 g 168 g
50 mm 65.4 cm³ 173 g 177 g 208 g 151 g 328 g
60 mm 113.1 cm³ 300 g 306 g 360 g 261 g 567 g
80 mm 268.1 cm³ 710 g 727 g 853 g 619 g 1,343 g
100 mm 523.6 cm³ 1,388 g 1,419 g 1,665 g 1,210 g 2,623 g

Two rules fall out of the chart. Every 25 percent increase in diameter roughly doubles the weight, and doubling the diameter multiplies it by eight. Fluorite runs about 20 percent heavier than quartz at the same size; pyrite almost twice as heavy.

Worked examples from real listings

Rose quartz, listed as 80 mm and 712 g. Radius 4.0 cm. 4.0³ = 64, × 4.18879 = 268.1 cm³, × 2.65 = 710 g. The listed 712 g agrees to within 2 g, which means both numbers were measured, not guessed.

Honey calcite, listed as 54 mm and 224 g. Radius 2.7 cm. 2.7³ = 19.68, × 4.18879 = 82.4 cm³, × 2.71 = 223 g. Another exact match.

Smoky quartz, listed as 60 mm and 260 g. A true 60 mm quartz sphere would weigh about 300 g. Working backwards, 260 g ÷ 2.65 = 98.1 cm³, which is the volume of a sphere about 57 mm across. The weight is the more trustworthy number; "60 mm" is a rounded size class. A strawberry quartz sphere listed as 55 mm and 260 g lands on the same 57 mm from the other side.

Amethyst, listed as 65 mm and 1 lb. A 65 mm quartz sphere weighs about 381 g, roughly 0.84 lb. The listing includes a glass stand, which accounts for the difference. Whenever a weight in pounds looks generous, ask whether the stand is included.

The Smithsonian crystal ball. The Penn Museum gives it as 12.875 inches across and 106.75 pounds. 12.875 inches is 32.70 cm, so the radius is 16.35 cm; 16.35³ × 4.18879 = 18,307 cm³, × 2.65 = 48,515 g, or 107.0 lb. The formula lands within a quarter of a pound of the published weight, a nice check that the ball is solid rock crystal.

How do you find the diameter from the weight?

Reverse the steps: divide the weight by the specific gravity to get the volume, divide by 4.18879, take the cube root to get the radius, and double it.

  • Fluorite sphere, 1.43 lb. 1.43 × 453.59 = 649 g. 649 ÷ 3.18 = 204.1 cm³. ÷ 4.18879 = 48.7. Cube root = 3.65 cm radius, so about 73 mm across.
  • Labradorite sphere, 1 lb. 454 g ÷ 2.69 = 168.8 cm³, giving a diameter of about 69 mm.
  • Rose quartz sphere, 3.04 lb. 1,379 g ÷ 2.65 = 520 cm³, giving almost exactly 100 mm.

If the computed diameter comes out well below the stated one, the stated figure is generous or the weight includes packaging or a stand.

Why don't clusters and specimens follow a formula?

Irregular pieces have no simple volume, and much of a specimen's weight can be matrix rock rather than the mineral named in the title. A 19 lb agate geode is mostly the dense outer rind; a cluster of apophyllite on stilbite weighs what its base weighs. For specimens, use the weight as a rough guide to how big the piece is and rely on stated dimensions (length × width × height) and photos with a scale for everything else. The crystal pricing guide explains why matrix weight is priced differently from crystal weight.

How big is a 60 mm sphere in the hand?

Sixty millimeters is 2.36 inches. A quartz sphere that size weighs about 300 g, about two thirds of a pound, and sits comfortably in a cupped palm. Below 40 mm, spheres are pocket and desk pieces. From 80 mm, two hands are wiser, and at 100 mm a quartz sphere weighs over 1.3 kg, enough to dent a wooden floor and to need a solid stand. Diameter in inches multiplied by 25.4 gives millimeters; in the other direction, divide by 25.4.

What to look for when you buy by size and weight

Prefer listings that give both a diameter and a stone weight, and run the chart check before paying. A close match suggests the seller measured the actual piece. A big gap means a rounded size class, an included stand, or a mistake. For specimens, look for three dimensions and a photo with a ruler or coin.

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Frequently asked questions

How many carats are in a gram?

Five. A metric carat is defined as 200 milligrams, so one gram is five carats and one kilogram is 5,000 carats. Carats are standard for cut gemstones; for spheres and specimens, grams are clearer and easier to compare.

How much does a 60 mm crystal sphere weigh?

It depends on the stone. A 60 mm quartz sphere weighs about 300 g, calcite about 306 g, fluorite about 360 g, selenite about 261 g and pyrite about 567 g. Multiply 113.1 cm³ by the stone's specific gravity for any other material.

Why is the listed weight different from what the formula gives?

The usual causes are a rounded size class, a stand or box included in the weight, pits or hollows in the stone, or a different material from the one named. A small difference of a few percent is normal; a big one deserves a question to the seller.

Is a pound of crystal the same as a pound of gold?

Not always in name. Crystals are sold in avoirdupois pounds of 453.59 g. Gold and silver use troy ounces of 31.10 g, and a troy pound of 12 troy ounces is about 373 g. A crystal listing in pounds or ounces almost always means avoirdupois.

How can I measure a sphere at home?

Weigh it on a kitchen scale in grams, then use the reverse method: divide by the specific gravity, divide by 4.18879, take the cube root and double it. For a direct check, place the sphere between two upright books, measure the gap, and repeat in two other directions.

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