Short answers, pulled from the story.
The first known classification of minerals dates back to 315 BCE when Theophrastus presented his treatise On Stones. This ancient attempt to organize the natural world laid the groundwork for a discipline that would eventually recognize over 6,145 distinct mineral species today.
Eight elements account for most of the key components of minerals and sum to over 98% of the crust by weight. These eight elements in order of decreasing abundance are oxygen, silicon, aluminum, iron, magnesium, calcium, sodium, and potassium.
The Mohs hardness scale measures resistance to scratching and is defined by ten indicators where a mineral with a higher index scratches those below it. The scale ranges from talc to diamond which is the hardest natural material.
Silicates are by far the most important class of minerals in terms of rock formation and diversity with over 90% of the Earth's crust composed of these minerals. Non-silicate minerals are of great economic importance especially as ores but are generally rare compared to silicates.
True minerals are solid substances with a fairly well-defined chemical composition and a specific crystal structure that occur naturally in pure form. Some natural solid substances without a definite crystalline structure such as opal or obsidian are more properly called mineraloids.