CGC Cheat Sheet 2026

The 30 highest-yield CGC facts, distilled from real exam questions. Print it, save it as a PDF, or study it here — free, no sign-up.

1. Which gem is known for a sharp diagnostic absorption line at 653.5 nm in the spectroscope? → Zircon
2. In Certified Gemstone Consultant, which factor MOST influences the selection of appropriate research methodology & design? → The specific requirements and constraints of the situation
3. Which data analysis & statistical methods method provides the MOST reliable results in Certified Gemstone Consultant? → Standardized protocols with validated measurement tools
4. When instrumentation & calibration in Certified Gemstone Consultant shows signs of wear, what is the CORRECT response? → Remove from service, tag out, inspect, and repair before returning to use
5. When testing whether heated and unheated sapphires of the same grade sell at different average prices, what is the null hypothesis? → There is no difference in mean price
6. A sapphire whose color was produced by beryllium diffusion must be disclosed because: → It significantly affects the stone's value compared with traditionally heated stones
7. What is the highest possible clarity grade for a gemstone? → Flawless
8. Garnets crystallize in which crystal system, making them singly refractive? → Isometric (cubic)
9. What is the most common style of ring setting used for engagement rings? → Prong setting
10. Which type of sapphire color commands the highest price per carat in the U.S. market? → Royal blue
11. In Certified Gemstone Consultant, how should data analysis & statistical methods results be communicated to stakeholders? → Through clear, structured reports with actionable recommendations
12. What happens to the standard error of a sample mean when the sample size increases? → It decreases
13. Two graders disagree on a diamond's clarity grade. What is the best QA response? → Have a third grader assess it blind, without seeing the earlier grades
14. Under the FTC Jewelry Guides, which condition does NOT on its own require a gemstone treatment to be disclosed? → The treatment was performed outside the United States
15. What is the main characteristic used to classify gemstones? → Chemical composition
16. Under the FTC Guides, if a diamond's weight is stated in decimal form (e.g., 1.50 ct), the figure must be: → Accurate to the last decimal place shown
17. What does 'liquidity' mean when applied to gemstone investments? → How easily a gemstone can be converted to cash at fair value
18. Which practice best guards against claims that a stone lost weight during repair? → Record the weight before and after service on the job ticket
19. What is the significance of 'origin' when pricing a colored gemstone? → Certain origins are associated with superior quality and command higher premiums
20. What is the most common treatment applied to blue sapphires to improve their color? → Heat treatment
21. What type of treatment is commonly used on emeralds to mask surface fractures and improve clarity? → Fracture filling with oil or resin
22. Under federal law, the lead content limit for substrate materials in children's jewelry is: → 100 ppm
23. What is the most important factor in determining the value of a diamond? → Cut
24. What is the primary consideration when designing a piece of jewelry for a client? → The client's design preferences
25. Immersion showing color concentrated at facet junctions and the girdle suggests which treatment in sapphire? → Surface diffusion
26. What documentation is ESSENTIAL for instrumentation & calibration management in Certified Gemstone Consultant? → Maintenance logs, calibration records, and incident reports
27. What is the FOUNDATION of effective quality assurance & control in Certified Gemstone Consultant? → Clearly defined standards and measurable criteria
28. What does the term 'fair trade' mean in the gemstone industry? → Ensuring fair wages and ethical working conditions
29. Which of these is an example of a sustainable practice in the gemstone industry? → Utilizing renewable energy in gemstone mining
30. A colorless stone reads RI 1.544–1.553, birefringence 0.009, and SG 2.65. What is it most likely? → Quartz