From m-w.com

Main Entry: 4rock
Function: noun
Etymology: Middle English rokke, from Old French dialect (Norman & Picard) roke, from Vulgar Latin *rocca
Date: 14th century
1: a large mass of stone forming a cliff, promontory, or peak
2: a concreted mass of stony material; also : broken pieces of such masses
3: consolidated or unconsolidated solid mineral matter; also : a particular mass of it
4 a: something like a rock in firmness: (1): foundation, support (2): refuge <a rock of independent thought…in an ocean of parochialism — Thomas Molnar> b: something that threatens or causes disaster —often used in plural
5 a: a flavored stick candy with color running through b: rock candy 1
6slang a: gem b: diamond
7 a: a small crystallized mass of crack cocaine b: crack 9
8: the ball used in basketball

Main Entry: 1stone
Pronunciation: \ˈstōn\
Function: noun
Etymology: Middle English, from Old English stān; akin to Old High German stein stone, Old Church Slavic stěna wall, and perhaps to Sanskrit styāyate it hardens — more at steato-
Date: before 12th century
1: a concretion of earthy or mineral matter: a (1): such a concretion of indeterminate size or shape (2): rock b: a piece of rock for a specified function: as (1): a building block (2): a paving block (3): a precious stone : gem (4): gravestone (5): grindstone (6): whetstone (7): a surface upon which a drawing, text, or design to be lithographed is drawn or transferred
2: something resembling a small stone: as a: calculus 3a b: the hard central portion of a drupaceous fruit (as a peach) c: a hard stony seed (as of a date)
3plural usually stone : any of various units of weight; especially : an official British unit equal to 14 pounds (6.3 kilograms)
4 a: curling stone b: a round playing piece used in various games (as backgammon or go)
5: a stand or table with a smooth flat top on which to impose or set type
— in stone : in or into a permanent and unchangeable state <plans are not set in stone>

Main Entry: 1min·er·al
Pronunciation: \ˈmin-rəl, ˈmi-nə-\
Function: noun
Etymology: Middle English, from Medieval Latin minerale, from neuter of mineralis
Date: 15th century
1: ore
2: an inorganic substance (as in the ash of calcined tissue)
3obsolete : mine
4: something neither animal nor vegetable
5 a: a solid homogeneous crystalline chemical element or compound that results from the inorganic processes of nature; broadly : any of various naturally occurring homogeneous substances (as stone, coal, salt, sulfur, sand, petroleum, water, or natural gas) obtained usually from the ground b: a synthetic substance having the chemical composition and crystalline form and properties of a naturally occurring mineral
6plural British : mineral water



By my reading, salt as a hard inorganic crystal definitely qualifies, but iron as a non-crystaline but inorganic substance may or may not qualify depending on which definition you look at.