The extracellular matrix consists primarily of calcium and phosphorus in the form of hydroxyapatite, Ca10(PO4)6(OH)2.
By definition all bone is hard. Before bone is fully formed it is made of cartilage and so is softer but it is not called bone at that time in development. Bone is made of hydroxyapetite mainly a calcium and phosphate salt. This makes it hard.
What tooth enamel is made of pretty much depends on who you ask. Some experts say, "Enamel's primary mineral is hydroxyapatite", to which I disagree because how could they know the enamel's composition if they can't even get the correct word usage right? Hydroxyapatite is the incorrect use of the word, "hydroxyL-apatite" (capital letter and hyphen used for clarification) but that is not what tooth enamel is made of either according to others who say, "tooth enamel is calcium hydroxy phosphate". Other so called experts [who claim that Fluoride (a corrosive toxic poison) is good for you] say that the tooth enamel's strength is "enhanced by the formation of fluoroapatite, a naturally occurring component of tooth enamel". This is also a lie because Fluoroapatite (more correctly spelled fluorapatite), is a natural calcium halophosphate or fluorinated calcium phosphate mineral that is mined from the earth for the making of phosphate fertilizer. Fluorine (used in some Freon) and Hydrofluosilicic acid (used to fluoridate drinking water and ther industrial applications) are both highly toxic and dangerous and are liberated from the natural mineral Fluorapatite. Fluorapatite is not a "naturally occurring component of tooth enamel". Still others argue that tooth enamel is, "calcium phosphate" (I agree) while other experts say enamel is, "composed of approximately 28% organic matter and 72% inorganic (calcium)", which is a vague but some-what true response although I have to plead ignorance on the actual percentages of calcium phosphate in the enamel. Everyone agrees that under the enamel is what's called, "dentin" but as for what the enamel is - well that depends on who your asking and whether or not they are trying to promote the fluoride myth. You see if they can get you to believe that your teeth "already contain naturally occurring fluoroapatite" (which they do not), then what's the harm in adding a little more fluoride to strengthen them? From the research and knowledge that I have come to find, my best answer is that tooth enamel is basically composed of calcium phosphate mineral but not hydroxylapatite.