7 (level of hardness)
You can determine the streak of a mineral whose Mohs scale is higher than the streak plate by either filing or crushing with a hammer before rubbing the sample on a streak plate.
Topaz (8), corundum (9), and diamond (10).
Granite is a hard igneous rock with a hardness of around 6-7 on the Mohs scale. Its streak is typically colorless, leaving no streak on a streak plate.
A mineral that can scratch glass but not a streak plate is quartz. Quartz has a hardness of 7 on the Mohs scale, which allows it to scratch glass (which has a hardness of about 5.5) but is harder than the streak plate, typically made from porcelain, which has a hardness of around 6-7. Therefore, while quartz can leave scratches on glass, it will not produce a streak on a streak plate.
Those minerals that are harder than the unglazed porcelain streak plate will scratch it rather than leave a streak.
You can determine the streak of a mineral whose Mohs scale is higher than the streak plate by either filing or crushing with a hammer before rubbing the sample on a streak plate.
Topaz (8), corundum (9), and diamond (10).
Corundum has a streak that is typically colorless, making it difficult to determine on a streak plate. The hardness of corundum (9 on the Mohs scale) can also affect the accuracy of the streak test, as it can scratch the streak plate instead of leaving a distinct streak.
Granite is a hard igneous rock with a hardness of around 6-7 on the Mohs scale. Its streak is typically colorless, leaving no streak on a streak plate.
Minerals with a hardness greater than around 7 on the Mohs hardness scale will not leave a streak on a standard unglazed porcelain streak plate. They will instead scratch and powder the streak plate.
Diamond is the only mineral that can scratch glass but not leave a streak on a streak plate. Glass has a hardness of around 5.5 on the Mohs scale, while a streak plate typically has a hardness of around 6.5. Diamond, with a hardness of 10, is able to scratch glass but not the streak plate.
Porcelain has a Mohs hardness of approximately 6-7, making it a relatively hard material. This hardness allows porcelain to be used as a streak plate in mineral testing to determine the color of a mineral's powder when scratched against it.
Diamond will not leave a streak on a porcelain streak plate because diamond is harder than the streak plate. It will leave a scratch on the streak plate for the same reason.
Two minerals that do not leave a streak on a streak plate are quartz and fluorite. Both minerals have a hardness higher than that of the streak plate, so they will not leave a streak when rubbed against it.
A mineral that can scratch glass but not a streak plate is quartz. Quartz has a hardness of 7 on the Mohs scale, which allows it to scratch glass (which has a hardness of about 5.5) but is harder than the streak plate, typically made from porcelain, which has a hardness of around 6-7. Therefore, while quartz can leave scratches on glass, it will not produce a streak on a streak plate.
Those minerals that are harder than the unglazed porcelain streak plate will scratch it rather than leave a streak.
A streak test is not used to identify minerals with a hardness greater than 7 on the Mohs scale, as these minerals can scratch the streak plate. Additionally, streak tests may not be effective for identifying minerals that have a streak color similar to the streak plate itself.