I. Observe Color and Texture (Most Intuitive)
High-quality Precision Grade Black Granite (Represted by Jinan Qing)
Color: Pure black, uniform, matte black, not shiny
Grain: Extremely fine, dense, particles almost invisible to the naked eye
Texture: No obious streaks, no white spots, no mica highlights
Fracture: Fine fracture, like ink jade, not rough
Ordinary Black Granite
Color: Graish, bluish, brownish, or mixed black and white
Grain: Coarse, visible quartz/feldspar particles
Common features: White highlights, mica reflections, st color spots
Fracture: Granular, loose
Mnemonic: The blacker, finer, and denser, the more suitable for precision; the moreled, coarser, and shinier, the less suitable for precision.
II. Feel Density and Weight (Most Reliable)
Precision grade black granite has extremely high density and is significantly heavier same volume.
High-Quality Precision Stone
Density: 2.9–3.1 g/cm³
Hand feel: Heavy, weighty
Tapping ssp, metallic, long resonance
Ordinary Granite
Density: 2.6–2.7 g/cm³
Hand feel: Light
Tapping sound: Mued, dull, no resonance
III. Check Water Absorption Rate (Determines Long-Term Stability)
Precision measuring platforms must have extremely low water absorption rate; otherwise, oil and water absorptio deformation.
Simple Test
Drop a drop of water on the surface
High-quality stone: Water droplet remains intact, no seepage, no spreading, no absorptdinary stone: Spreads quickly, seeps, darkens
Precision Grade Standard: Water absorption rate < 0.01%
Ordinary granite is genrally 0.1%–0.5%, an order of magnitude higher.
IV. Check for Mica, Quartz Bright Spots (Fatal Defects)
Precision platforms strictly prohibit mica and brght spots.
Mica: Will flake off, chip, and wear unevenly
Bright quartz spots: Extremely high hardness, will protrude during grinding, destroying flatness
High-quity Stone
No flashing particles
Uniform jet black overall
Ordinary Stone
Large amounts of white/silver flashing points
Especially obvious under sunlight
V. Hardness and Wear Resnce (Key for Precision Grinding)
Precision granite must have high and uniform hardness.
Testing Method
Scratch with a key or knife:
High-quality: Cannot be scratched at all, rks
Ordinary: Easily leaves white marks
Mohs Hardness:
Precision Grade: 6.5–7.5
Ordinary: Mostly 5–6
VI. ernal Defects (Visual Inspection Simple Tapping)
Precision platforms must not have cracks, hidden cracks, or layers.
High-quality: Tapping sound is uniform and crisp
With hdden cracks: Sound is dull, intermittent, and broken
Surface must not have:
Cracks
Repair marks
Color difference from glue repair
Holes
VII. Thermal Staty (Core for Precision Measurement)
Ordinary granite has a large and uneven coefficient of thermal expansion, deforming with temperature changes.
High-Quality Precision Granite
Expansion coeffcient: 4.5–5.5×10⁻⁶ /°C
Material is uniform, good isotropy
Minimal deformation under temperature changes
Ordinary Granite
Expansion coefficient is high astable
Large internal particle differences, prone to local warping
VIII. Industry-Accepted Grades (Direct Comparison)
Stones suitable for precision measuring platforms
Jina — Top grade, first choice for global precision equipment
Zhangqiu Black
Some Special Grade Mongolian Black (requires strict screening)
Ordinary granite unsuitable for precision meaement
Sesame Black, Fuding Black, Shanxi Black, Guangxi Black, etc. (building stones)
Any black granite with patterns, spots, or streaks
IX. Quick Summary: 3-Second Identification Method
Check the color: Pure black without impurities = Good; Grayish or mottled or
Check the texture: Invisible crystal grains = Good; Obvious particles = Poor
Check the weight: Heavy and solid = Good; Light and floaty = Poor
Check the d: Clear and long resonance = Good; Dull = Poor
Check water absorption: Non-absorbent = Good; Absorbs and spreads = Poor
If all 5 critria are met, it is basically precision-grade black granite.
