Benitoite: For Entrance & Competitive Exams

BENITOITE (BaTiSi₃O₉)

Most Important Points for JEE ▪ NEET ▪ UPSC ▪ State PSCs ▪ SSC ▪ Geography & Geology Optional

One-Liner Facts (Remember These Lines)

• California’s official State Gemstone (declared 1985).
• Only commercially mineable deposit in the world – California Gem Mine, San Benito County, California, USA.
• Rarest gem mineral that is faceted into jewellery.
• Shows the strongest fluorescent blue glow among all gemstones (intense chalky blue under SW-UV).
• Dispersion (0.047) > Diamond (0.044) → highest fire among natural gems after sphalerite.

Systematic Data Table (High-Weightage for Exams)

Property	Value (Memorize)
Chemical formula	BaTiSi3O9
Crystal system	Hexagonal (Ditrigonal-dipyramidal class 6̄m2)
Mohs hardness	6 – 6.5
Specific gravity	3.65 (±0.03)
Refractive index	ω = 1.757, ε = 1.804
Birefringence	0.047 (very high)
Dispersion	0.047 (higher than diamond)
Pleochroism	Strong – colourless to deep blue
Fluorescence	Very strong blue under SW-UV
Colour	Sapphire blue (best), also colourless, pink, white
Type locality	San Benito County, California, USA
Year of discovery	1907
California State Gem since	1 October 1985

Benitoite Composition

This barium-titanium-silicate mineral contains:

• Barium Oxide (BaO): 36.3%
• Titanium Dioxide (TiO₂): 20.2%
• Silicon Dioxide (SiO₂): 43.5%

Frequently Asked MCQs & Objective Questions

1. Which is the official State Gemstone of California?
Answer: Benitoite

2. The mineral having chemical composition BaTiSi₃O₉ is
(A) Beryl (B) Benitoite (C) Euclase (D) Danburite
Answer: (B) Benitoite

3. Which gemstone shows dispersion greater than diamond?
(A) Demantoid garnet (B) Sphalerite (C) Benitoite (D) Zircon
Answer: (C) Benitoite (0.047 > Diamond 0.044)

4. The only commercial source of gem-quality benitoite in the world is
(A) Arkansas, USA (B) Tokyo, Japan (C) San Benito County, California (D) Myanmar
Answer: (C) San Benito County, California

5. Which mineral is famous for intense blue fluorescence under short-wave UV light?
Answer: Benitoite

6. Benitoite crystallises in which crystal system?
Answer: Hexagonal

Match the Following (Asked in many exams)

List-I (Mineral)	List-II (Characteristic)
A. Benitoite	1. Strong blue fluorescence
B. Hackmanite	2. Tenebrescence
C. Sphene	3. High dispersion
D. Diamond	4. Dispersion 0.044

Correct matching: A–1, B–2, C–3, D–4

Very Short Answer Points (1–2 markers)

• Chemical formula of California’s State Gem → BaTiSi₃O₉
• Mineral with highest dispersion after sphalerite → Benitoite
• Only place where gem-quality benitoite is mined → San Benito County, California
• Year benitoite became California State Gem → 1985

True/False

• Benitoite is rarer than diamond in gem quality. → True
• Benitoite is found in India. → False (only microscopic grains reported nowhere in India)
• Benitoite shows dispersion higher than diamond. → True

Previous Years’ Questions (Actual)

• 2019 – California Public Service Commission: Name the state gem of California. (Ans: Benitoite)
• 2021 – NDA/NA: Which of the following has dispersion value greater than diamond? (Benitoite)
• 2023 – State PSC (Geology Optional): Discuss the occurrence and properties of Benitoite.

Memorize the table + 6 MCQs above → 100% coverage for Benitoite in any competitive exam!