(Yikes! Carnotite is a uranium mineral and it's radioactive!) "Rock" can be a mass noun (uncountable noun). It can also be a countable noun. In your case you want to say "Navajos once used carnotite, a powdery yellow rock, in the creation..." Here are some examples of use: a) Granite is a hard rock, shale is a soft rock. b) He was throwing rocks at a cat, and I told him to stop. c) She has an interesting rock collection. d) I can't let myself go into a rock shop, because I can't resist buying the lovely rocks and minerals. e) New Hampshire license plates bear an image of "The Old Man of the Mountains," a famous rock formation. I'm very sorry about the next pair! They mean the same thing! The first puts the emphasis on the separate, individual rocks, the second on the mass. f) Behind the barn, there was a huge pile of rocks. g) Behind the barn, there was a huge pile of rock. The difference in emphasis is clearer in these two sentences: h) Behind the barn, there was a huge pile of over a thousand rocks. i) Behind the barn, there was a huge pile containing several tons of rock. j) I didn't have a hammer, so I pounded the stake in with a rock.

.... and I'm presuming that you meant 'indefinite article', rather than 'adjective'. If you are referring to the 'a', then yes, you are right. You can't have a plural noun following the article 'a'. The noun should be 'rock', in the singular.

Not always true, but in this sentence "rocks" should lose the s and be the singular "rock" However, this is because of the article "a", not the reason you stated. For example: "There is a big, green tree over there". (1 tree, so singular) "There are thousands of big, green trees over there". (More than 1 tree, so plural).

It should be 'rock', not 'rocks'.