1. Color:

* Quartz: Typically clear, white, or milky, but can also be smoky, rose, or amethyst colored.

* Plagioclase: Usually white, gray, or off-white, but can have a greenish or bluish tinge.

2. Cleavage:

* Quartz: No cleavage - it breaks irregularly (conchoidal fracture).

* Plagioclase: Excellent cleavage in two directions, creating flat, smooth surfaces.

3. Hardness:

* Quartz: Hard (7 on Mohs hardness scale). It can scratch glass.

* Plagioclase: Less hard than quartz (6 on Mohs hardness scale). It won't scratch glass.

4. Luster:

* Quartz: Glassy luster.

* Plagioclase: Generally duller, more pearly or sub-vitreous luster.

5. Crystal Shape:

* Quartz: Can have a hexagonal crystal shape, but often occurs in irregular grains.

* Plagioclase: Typically appears as rectangular or blocky crystals.

6. Specific Gravity:

* Quartz: Specific gravity of 2.65.

* Plagioclase: Specific gravity ranges from 2.62 to 2.76.

7. Additional Tips:

* Presence of Striations: Plagioclase often shows fine parallel lines called "striations" on its cleavage faces. Quartz does not have striations.

* Location in the Rock: In igneous rocks, quartz is often found in the later stages of crystallization and can be found in the groundmass. Plagioclase is typically an early-forming mineral and can be found as large crystals within the rock.

Note: It's best to use a combination of these characteristics to make a positive identification. If you are unsure, it's always a good idea to consult with a geologist or mineralogist for confirmation.