Tanzanite vs. Iolite: A Gemstone Comparison

Tanzanite and Iolite are two beautiful blue-violet gemstones that can occasionally be mistaken for one another, especially in their deeper hues. While both can exhibit striking purplish-blue tones and strong pleochroism, their mineral composition, formation, and value are very different.

Tanzanite is a relatively new gemstone on the market, discovered only in the 1960s, yet it quickly rose to fame for its captivating colour and limited geographic origin. Iolite, meanwhile, has a longer historical presence, known for centuries and once referred to as the “Viking’s Compass” due to its optical properties.

Let’s dive into the key distinctions between Tanzanite and Iolite, from their gemmological properties to their symbolic meanings.

How to Tell the Difference: Visual & Gemmological Tests

Visual Clues

Tanzanite: Rich violet-blue to purplish-blue, with some stones appearing more purple or more blue depending on the lighting and angle. Often transparent with good clarity, especially in faceted stones.

Iolite: Deep blue to violet-blue with a greyish or slightly inky undertone. Pleochroism is strong, but colours may appear more subdued compared to Tanzanite. Although eye visable inclusions are common, faceted stones can also be eye clean, but may have colour zoning.

Gemmological Tests

Test	Tanzanite	Iolite
Refractive Index (RI)	1.691 - 1.700 (+/- 0.005)	1.522 - 1.596
Birefringence	0.008 - 0.013	0.005 - 0.018
Pleochroism	Strong 3 Colours (violet, blue, burgundy)	Strong 3 Colours (blue, violet, pale yellow/grey)
Optical Character	Doubly refractive Biaxial positive	Doubly refractive Biaxial positive or negative

 

Colour

Tanzanite: Colour ranges from violet-blue to deep purple-blue. Natural tanzanite tends to be more brownish before heating, as such most tanzanite gemstones are heat treated.

Iolite: Displays a greyish-blue to violet hue, often with a steely or smoky quality. Its pleochroism can make it flash yellow-grey or blue depending on the viewing angle.

Clarity & Inclusions

Tanzanite: Typically eye clean (Type 1). If it does have inclusions they'll usually be needle-like crystals or liquid inclusions. 

Iolite: Typically has eye visible inclusions (Type 2), although you can readily get eye clean faceted gemstones too. May have plate-like inclusions or internal zoning.

Gemstone Formation

Tanzanite: A variety of the mineral zoisite, formed in metamorphic rocks under high pressure and heat. Found exclusively in the Merelani Hills of northern Tanzania.

Iolite: A variety of the mineral cordierite, found in metamorphic and igneous rocks. Mined in India, Sri Lanka, Brazil, and parts of Africa.

Durability & Wearability

Scratch Resistance (Mohs Hardness Scale):

Tanzanite: 6 to 7 (Slightly less hard than common household dust so can get quiet scratched over time with everyday wear)

Iolite: 7–7.5 (Around the same hardness as common household dust so fine for everyday wear)

Cleavage & Toughness:

Tanzanite: One direction of perfect cleavage and fair toughness, meaning it can break if struck or exposed to rough handling.

Iolite: Distinct cleavage in one direction and fair to good toughness. Slightly more durable than Tanzanite, but still needs caution under impact.

Best Use: Tanzanite is best suited for earrings, pendants, or rings with protective settings that aren’t worn daily. Iolite, being slightly harder, is more suitable for everyday rings but will still need a protective setting.

Enhancements & Treatments

Tanzanite: Almost always heat treated to enhance its colour (this is stable and widely accepted). Untreated stones are very rare and usually brownish.

Iolite: Generally untreated. Its natural colour and clarity are typically considered sufficient without enhancements.

Both can be lab-grown, but synthetic Tanzanite is more commonly seen on the market than synthetic Iolite.

Price Comparison

Tanzanite: Retail prices vary from £100 per carat for small, lower-grade stones to over £1,500 per carat for large, intensely coloured, high-clarity gems.

Iolite: Far more affordable, usually ranging from £5 to £100 per carat depending on size and quality.

Size

Tanzanite: Commonly available in sizes from 0.1 to 10 carats, with larger stones up to 20+ carats available, though high-quality large stones are rare and expensive.

Iolite: More readily available than tanzanite in a wide range of sizes, including larger faceted stones. Stones from 0.25 to 10 carats are common and relatively affordable.

Rarity & Sourcing

Tanzanite: Extremely rare due to its single-source origin in Tanzania. Supplies are expected to deplete within the next few decades.

Iolite: More abundant and mined in several countries. Not considered a rare gemstone, but still offers excellent visual appeal.

Cut & Shape Preferences

Tanzanite: Common cuts include oval, cushion, pear, and emerald. The cut is often designed to highlight its pleochroic colour play and brilliance.

Iolite: Usually cut into ovals, rounds, or cushions. Cutters often orient the stone to maximise its best blue/violet face-up colour while balancing pleochroism.

Symbolic & Spiritual Associations

Tanzanite: Said to aid spiritual growth, transformation, and intuition. Also associated with calming energy and insight.

Iolite: Linked to vision, clarity, and self-discovery. Thought to have historically been used as a navigation aid and said to enhance inner strength and mental focus.

Comparison Summary

Feature	Tanzanite	Iolite
Hardness	6 to 7	7 to 7.5
Cleavage	Perfect in one direction	Distinct, but more durable
Colour	Blue-violet to violet-blue	Greyish-blue to violet
Commonly Included?	Usually eye clean	Usually included
Common Treatments	Heat treated	Rarely treated
Price per Carat	£100–£1,500+	£5–£100
Rarity	Extremely rare (single-source)	Relatively common
Large Sizes? (5 ct+)	Available but pricey	Readily available
Associated With	Intuition, transformation, calm	Vision, focus, inner strength

Tanzanite and Iolite both offer a beautiful violet-blue hue, but their differences in rarity, durability, and value make them distinct choices.

If you’re looking for a rare, luxurious stone with dramatic pleochroism and intense colour, Tanzanite is an unforgettable choice. But if you're after an affordable, durable gem with subtle colour shifts and historic charm, Iolite is a fantastic alternative.

Either way, both of these beauties would bring a unique touch of magic to your jewellery designs.