Learn Turkish: Bu katman mermerden ibaret. - This layer consists of marble.

The word ibaret means "consisting of" or "comprised of." In Turkish, it strictly requires the preceding noun to take the ablative case (-DAn). So, to express "consists of marble," you must say mermerden ibaret (literally "from marble consisting").

Does ibaret imply that the layer is only made of marble?

Yes, exactly. While it translates broadly to "consists of," ibaret carries an exclusive nuance, often meaning "is composed entirely of" or "is nothing but." If the layer were just partially made of marble mixed with other stones, using ibaret would sound incorrect.

Where is the verb in this sentence? Shouldn't there be an olmak or a "to be" suffix?

In Turkish, the third-person "to be" suffix (-DIr) is frequently omitted in everyday statements. You could say Bu katman mermerden ibarettir to give the sentence a more formal, encyclopedic, or factual tone, but simply ending with ibaret is grammatically complete and very natural.

Could I say mermerden yapılmış instead of mermerden ibaret?