Although I'm not that interested in gemstones, if I had to choose, I like Ttopaz since my favorite colour is orange. Although I'm not that interested in gemstones, if I had to choose, I like topaz since my favorite color is orange.

1. I write in American English, so I spell that word as "color"; British English would demand that you put the "u" back in the word. :) 2. "topaz" is a common noun, so you don't capitalize it. On that note: gem-quality topaz can actually come in a variety of colors (the light blue variant is my personal favorite), but samples with a brownish-orange tint are certainly frequently encountered.

Although I'm not that interested in gemstones, if I had to choose, I like Topaz since my favorite colour is orange. Although I'm not that interested in gemstones, if I had to choose, I like Topaz since my favorite colour is orange.

Though "if I had to choose, I like Topaz..." isn't necessarily incorrect, it sounds unnatural. The more common phrase is. "if I had to choose, I would say..." so a more natural sounding sentence might be "Although I'm not that interested in gemstones, if I had to choose, I would say Topaz, since my favorite colour is orange."

Although I'm not that interested in gemstone, iIf I had to choose, I like Topaz since my favorite colour is orange. However, I’m not too interested in gemstones. If I had to choose, I like Topaz since my favorite color is orange. However, I’m not too interested in gemstones.

Although I'm not that interested in gemstone, if I had to choose, I likwould choose Topaz since my favorite colour is orange. Although I'm not that interested in gemstone, if I had to choose, I would choose Topaz since my favorite colour is orange.

Gemstones like diamonds are beautiful, but I don't think they have a worth more than hundreds of millions of yen. Gemstones like diamonds are beautiful, but I don't think they have a worth more than hundreds of millions of yen.

This is okay as is, but I also wanted to point out an alternative rephrasing: "I don't think it's worth spending hundreds of millions of yen on them."

Gemstones like diamonds are beautiful, but I don't think they have a worth of more than some hundreds of millions of yen. Gemstones like diamonds are beautiful, but I don't think they have a worth of more than some hundreds of millions of yen.

A slightly more natural way to say the second part of this sentence is: "but I don't think they're worth more than a few hundred-million yen"

Gemstones like diamonds are beautiful, but I don't think they have a worth more than hundreds of millions of yen. Gemstones like diamonds are beautiful, but I don't think they have a worth more than hundreds of millions of yen.

Gemstones like diamonds are beautiful, but I don't think they have ashould worth more than hundreds of millions of yen. Gemstones like diamonds are beautiful, but I don't think they should worth more than hundreds of millions of yen.

Do you mean: they shouldn't be as expensive?

RA replica is enough for me. A replica is enough for me.

Replica iss are enough for me. Replicas are enough for me.

Since you are speaking about replicas in general, you would use the plural form. Otherwise it would imply that you are only talking about 1 specific replica.

Replica is good enough for me. Replica is good enough for me.

Sounds a bit more natural. Not grammatically incorrect though!