Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Material | Formula | Refractive\nindex(es)\n589.3 nm | Dispersion\n431 – 687 nm | Hardness\n(Mohs'\nscale) | Density\n(g/cm3) | Thermal\nCond. | State of\nthe art |
| --- | --- | --- | --- | --- | --- | --- | --- |
| Diamond | C | 2.417 | 0.044 | 10 | 3.52 | Excellent | (Natural) |
| Glasses | Silica with Pb, Al, &/or Tl | ~ 1.6 | > 0.020 | < 6 | 2.4 – 4.2 | Poor | 1700 – |
| White Sapphire | Al2O3 | 1.762 – 1.770 | 0.018 | 9 | 3.97 | Poor | 1900–1947 |
| Spinel | MgO·Al2O3 | 1.727 | 0.020 | 8 | ~ 3.6 | Poor | 1920–1947 |
| Rutile | TiO2 | 2.62 – 2.9 | 0.33 | ~ 6 | 4.25 | Poor | 1947–1955 |
| Strontium titanate | SrTiO3 | 2.41 | 0.19 | 5.5 | 5.13 | Poor | 1955–1970 |
| YAG | Y3Al5O12 | 1.83 | 0.028 | 8.25 | 4.55 – 4.65 | Poor | 1970–1975 |
| GGG | Gd3Ga5O12 | 1.97 | 0.045 | 7 | 7.02 | Poor | 1973–1975 |
| Cubic Zirconia | ZrO2(+ rare earths) | ~ 2.2 | ~ 0.06 | ~ 8.3 | ~ 5.7 | Poor | 1976 – |
| Moissanite | SiC | 2.648 – 2.691 | 0.104 | 8.5–9.25 | 3.2 | High | 1998 – |
| Quartz | Silica | 1.543 – 1.554 |  | 7- | 2.50 – 2.65 |  | Ancient |
Question:
how hard is a diamond?
Answer:
10