Diabase or dolerite is a type of igneous rock that is composed primarily of plagioclase feldspar, pyroxene, and sometimes olivine. Plagioclase feldspar, which is a type of feldspar that contains calcium and sodium, is typically the most abundant mineral in diabase.

In some cases, diabase can display white lines or streaks that are composed of plagioclase feldspar. These white lines are commonly referred to as feldspar laths or feldspar porphyroblasts, and they are often visible to the naked eye.

The formation of these white lines is typically the result of a process known as crystal growth or crystallization. When diabase cools and solidifies from a molten state, the plagioclase feldspar crystals grow at different rates, with some crystals growing faster than others. This can lead to the development of large, well-formed feldspar crystals that are visible as white lines within the rock.

The presence of feldspar laths or porphyroblasts in diabase can be useful for geologists in identifying and classifying the rock. It can also provide insights into the conditions under which the rock formed, such as the rate of cooling and the presence of other minerals or substances in the magma