Quartz is a common mineral in many metamorphic and felsic igneous rocks that, like the potassium feldspars, also has a non-metallic luster and hardness greater than glass. Unlike the potassium feldspars, however, quartz does not exhibit cleavage, which easily distinguishes it from feldspar minerals.
https://www.esci.umn.edu/courses/1001/minerals/potassium_feldspar.shtml
Formation of Feldspar. Feldspars, the most important of the rock-forming minerals, are integral constituents of almost every igneous rock; in fact, it is the color of the feldspar that usually determines the color of the igneous rock of which it is a part.
http://www.gemstones-guide.com/Feldspar.html
Formula: Na2O.Al2O3.6SiO2. Alternate Names: Na-Feldspar, Na Feldspar, Albite. Soda Feldspar is generally a clean white mineral that sources sodium, alumina and silica along with some potassium and calcium. No actual materials have the ideal formula shown here, they all have some K2O and small amounts of other contaminants.
https://digitalfire.com/4sight/material/soda_feldspar_1264.html
It is far more difficult to discriminate between the feldspar groups internal divisions  to distinguish plagioclase minerals from potassium feldspars. The presence of fine parallel striations on some plagioclase cleavage surfaces is the best way to distinguish the two.
https://www.esci.umn.edu/courses/1001/minerals/potassium_feldspar.shtml
Sources of Feldspar. Historically, the most important source of moonstone has been Ceylon, where it is found in many areas as rolled, rounded pebbles in the gem gravels with spinel and corundum. In addition, it occurs in dikes in several places on Ceylon, notably in the central and southern provinces.
http://www.gemstones-guide.com/Feldspar.html
Feldspars are used widely in the glass and ceramics industries. Alkali feldspars are more commonly used commercially than plagioclase feldspars. Albite, or soda spar as it is known commercially, is used in ceramics. The feldspar-rich rocks larvikite and a few anorthosites are employed as both interior and exterior facing slabs. In addition, several feldspars are used as gemstones. For example, varieties that show opalescence are sold as moonstone.
https://www.britannica.com/science/feldspar/Uses
In almost every case it is a mixture of two or more types of feldspars. The moonstone effect, which is called ADUIARESCENE (ad-u-lar-ESS-ence), is caused by a concentration of the soda-type feldspar (albite) in oriented positions within the main mass of orthoclase.
http://www.gemstones-guide.com/Feldspar.html
Definition of feldspar. : any of a group of crystalline minerals that consist of aluminum silicates with either potassium, sodium, calcium, or barium and that are an essential constituent of nearly all crystalline rocks.
https://www.merriam-webster.com/dictionary/feldspar
In Our Society: The Economic Importance of Potassium Feldspar. Potassium feldspars are a significant component of many felsic igneous rocks used as decorative building stones. By volume, however, the most significant use of potassium feldspar is as aggregate in concrete and asphalt.
https://www.esci.umn.edu/courses/1001/minerals/potassium_feldspar.shtml
Check your grammar now! : any of a group of crystalline minerals that consist of aluminum silicates with either potassium, sodium, calcium, or barium and that are an essential constituent of nearly all crystalline rocks.
https://www.merriam-webster.com/dictionary/feldspar
what is feldspars sources
Feldspars sources is the most  important source of moonstone has been Ceylon, where it is found in many areas as rolled, rounded pebbles in the gem gravels with spinel and corundum.