Because it is a low density single crystal.
Dongling stone is an oil-green quartzite containing chromite, which has placer gold effect. There are lines with primary stone texture, which is not difficult to distinguish. Polished Dongling stone similar to sapphire can often be seen in the market. Although the hardness of Dongling stone is high and it is easy to crack after collision, it is usually recognized at a glance because it looks dry and has no oiliness of Hetian jade.