did he study rocks or precious stones?
Yes, he classified rocks and gems based on their behavior when heated.
Theophrastus
In his treatise "On Stones" (Περὶ λίθων), which was to be used as a source for other lapidaries until at least the Renaissance, Theophrastus classified rocks and gems based on their behavior when heated, further grouping minerals by common properties, such as amber and magnetite, which both have the power of attraction. 
where do birds go when they fly south The southern oceans. what is this phenomenon called? It is the regular seasonal movement, often north and south along a flyway, between breeding and wintering grounds. do changes in climate affect this migration? Yes, large scale climatic changes are expected to have an effect on the timing of migration. are there any scientists who observed or studied this phenomenon? Yes, Johannes Leche. historically, what were some of the views on this phenomenon? Aristotle noted that cranes traveled from the steppes of Scythia to marshes at the headwaters of the Nile. who is this person mentioned here? A Greek philosopher and polymath during the Classical period in Ancient Greece. what has he said about the human soul? The human soul incorporates the powers of the other kinds: Like the vegetative soul it can grow and nourish itself. who was the philosopher that took over after him? Theophrastus when was this person born? c. 371 where was he born? Eresos in Lesbos.
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Theophrastus [SEP] On stones
In his treatise "On Stones" (Περὶ λίθων), which was to be used as a source for other lapidaries until at least the Renaissance, Theophrastus classified rocks and gems based on their behavior when heated, further grouping minerals by common properties, such as amber and magnetite, which both have the power of attraction. He also comments on the different hardnesses of minerals. Theophrastus describes different marbles; mentions coal, which he says is used for heating by metal-workers; describes the various metal ores; and knew that pumice-stones had a volcanic origin. He also deals with precious stones, emeralds, amethysts, onyx, jasper, etc., and describes a variety of "sapphire" that was blue with veins of gold, and thus was presumably lapis-lazuli.