天然 and 自然: natural fragrance and a natural manner of speaking
Compare 天然 describing fragrance and a cave with 自然 for nature in the mountains and 自然に speaking to guests. Distinguish origin, an object of experience, and manner.
Fragrance's origin and an untouched cave
天然（てんねん） means natural or untouched. 「この石けんには天然の香料が使われている。」（This soap contains natural fragrance.） 天然の describes the fragrance. Its natural origin does not add a guarantee of health, safety, or quality.
「この洞窟は天然のもので、人の手は加わっていない。」（This cave formed naturally and has remained untouched by people.） この洞窟 limits the claim to this cave, and 人の手は加わっていない describes it as untouched by people. The cave's stated condition does not become the preservation status of every natural object.
Nature experienced and a manner of speaking
自然（しぜん） means nature or naturalness. 「休日は山へ行き、豊かな自然に触れた。」（On my day off, I went to the mountains and spent time surrounded by nature.） 豊かな自然 is what was encountered in the mountains. 触れた describes that past experience of nature.
「彼女は初対面の客とも自然に話していた。」（She spoke naturally even with guests she had just met.） 自然に describes the manner of 話していた. The sentence gives her natural way of speaking even with guests she had just met. Its focus is manner, rather than the natural origin of a substance.
The object and manner identify the sense
天然の香料 describes fragrance's origin, and 天然のもの describes the naturally formed cave. 豊かな自然 names the nature experienced, while 自然に qualifies speaking. Identify what the word describes to distinguish object from manner.
The conversation example supplies no guarantee of honesty or of feelings being fully disclosed. Preserve the natural manner of speaking without replacing it with an unprocessed substance or a promise that the statements were truthful.
Do 「この石けんには天然の香料が使われている。」 and 「彼女は初対面の客とも自然に話していた。」 have the same focus?
Show the answer and why
The first describes the fragrance's natural origin. The second describes a natural manner of speaking with guests she had just met. That manner is neither a material's origin nor a guarantee of honesty.