Translation:Diamonds are very expensive.
Replying to the other two replies here the first diamond is an adjective referencing the unstated noun drill bit not a stand-alone noun itself.
Diamond is a singular noun the collective being cluster. Metals like Cobalt have no colletive.
Diamonds are expensive. The diamond is expensive. I bought an expensive cluster of diamonds.
Cobalt isn't counted the same way as diamond - When you have a chunk/piece of cobalt, you can call it a "chunk of cobalt" or a "piece of cobalt", but when you have a piece of diamond, you simply call it a "diamond". Likewise, you'd refer to pieces of cobalt in plural as "pieces/chunks of cobalt", but pieces of diamond as "diamonds". Sure, they belong to the same category of minerals in science, but in language, they belong to different grammatical categories.
If you had listened to what I said, you would've noticed that like the original comment and the lesson itself, I was referring to individual pieces of diamond - diamonds. Not the material "diamond" itself. The point is that diamond can be referred to both in pieces as "diamonds", and as the material, "diamond". The comparison to cobalt they made therefore doesn't work, as cobalt is referred to in pieces as "pieces/chunks of cobalt", whereas normally pieces of diamond are referred to simply as "diamonds". "One diamond" - "Two diamonds". I'm not saying that diamond isn't a continuous substance, I'm saying that it's BOTH, depending on context, and in this context, it's not referred to as a continuous substance.
Oh sorry I had misunderstood :) you mean the guy who said about the truckload of cobalt? Yes of course you can say a truckload of diamonds, wish I had one! I think generally, in this world where English is used internationally in many varieties, it is better to say you can say X rather than you cannot say X (because it doesn't confirm to my variety of English). E.g. just because "I know lah" isn't the Queen's English doesn't mean it is wrong - understood very well by Malaysians (I'm not one but know many). Peace! ;-D
You cannot tell from the standalone sentence what the context is. That is the point and an important one for learners of Japanese to understand. You have to let go of ingrained assumptions connected to your mother tongue. There is nothing that gives context that there are "pieces of diamond" here. If I say cobalt it could mean a piece of, or pieces of, or the element, or the colour cobalt. The only possibility, and I don't know but it could be the case, is that ダイヤモンド only refers to pieces of diamond, and then perhaps only the cut variety, and 金剛 or 金剛石 is used for the mineral and perhaps uncut diamonds. That would be the sort of thing I would research with native speakers before making an assumption. You know what they say about assumptions...
Your previous replies made it sound like you agreed with the other guy saying that the ONLY correct option is to refer to it as a continuous substance. If you however agree with me that both options are correct in the context, then it was just a mutual misunderstanding to begin with. I certainly was not trying to say that only one option is correct. I was in fact trying to say the exact opposite, to the guy who was saying diamonds can never be referred to as diamonds.
From what I know, 菱 by itself actually refers to a very specific plant: Trapa japonica.
Here's a page with some pictures of the Trapa genus (the first entry is on Trapa japonica): http://waterplants.web.fc2.com/fieldnote_vol11.html
Dried Trapa japonica pods were also used in feudal Japan as a natural caltrop to supplement supplies of metal caltrops. These were referred to as "天然菱" (てんねんびし)
‘’Diamond is expensive” is not proper Engrish. I’ve seen a lot of things like this in the Japanese that make me question if there are native English speakers on the team, and often it’s the English, and not the Japanese that trips me up, especially when I’m forced to write bad English to get an answer right.