It leaves a scratch instead of a streak because Topaz has a higher number on the Moh's scale compared to the 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.
Gold and pyrite leave a streak because they are both metallic minerals, which means they can rub off on a streak plate, leaving a colored line that reflects their metallic composition. In contrast, quartz is a non-metallic mineral with a hardness of 7 on the Mohs scale, making it too hard to leave a streak when rubbed against a streak plate. As a result, quartz typically does not produce a streak.
A streak plate
A streak plate, or unglazed white porcelain tile, is used to perform streak tests which can help to identify minerals. Rubbing the mineral on the streak plate will finely powder it and reveal the color of the powder, which in some cases is quite different from the color of the mineral. Eg: hematite is black but has a red streak - a good way to tell you have hematite instead of one of the many other shiny black minerals.
Streak color is determined by scraping the mineral across a a streak plate, (which is made of unglazed porcelain), and then observing the color of the streak, which is left on the plate. Note that some minerals do not leave a streak, as they are too hard. Thus, it is important to learn other identification methods, to use in conjunction with streak color, in order to identify minerals.
The lack of a streak would indicate that the mineral is harder than the streak plate, or the color of the streak is the same as the color of the streak plate.
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.
The streak of Jade is white.
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.
A streak plate is used to determine the streak colour of a mineral. You take the mineral and scratch it against the streak plate, and it will create a coloured streak on the plate. The streak is not always the same colour as the mineral itself.
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.
A mineral with a hardness of 9, such as corundum, would not leave a streak on a streak plate because it is harder than the plate itself, which typically has a hardness of about 6. A harder mineral cannot scratch a softer material, so it wouldn't produce a streak. Streak is determined by the powder left behind when a mineral is scraped against a harder surface, and since the mineral is too hard, it cannot do so.
Very hard minerals like diamonds can leave no streak because their hardness prevents them from being scratched by the streak plate. Since the streak test involves rubbing the mineral against a surface to leave a streak, extremely hard minerals won't leave a streak behind because they are tougher than the testing material.
The streak of a mineral refers to the color left on a streak plate after rubbing a mineral across its surface. A streak plate can be as simple as the unglazed side of a porcelain tile. The streak is the mineral in a powdered form from abrasion with the hard, slightly textured surface of the unglazed porcelain streak plate. The streak color may differ from the color observed in a specimen. The streak is used as an aid in the identification of minerals.
A white tile plate is used. Some minerals leave a streak; others don't.
A ruby's streak color appears colorless because as a mineral, ruby typically does not leave a streak when rubbed against a streak plate. The color of a mineral's streak is caused by the powdered form of the mineral, revealing its true color when scratched against a ceramic plate, but in the case of ruby, its hardness and structure prevent it from leaving a visible streak.
Mineral streak is the color of the powder produced when a mineral is scratched on a porcelain streak plate. It is a useful property for identifying minerals, as different minerals can leave distinctively colored streaks.