Hi! My name is Thomas, I am a senior student in Canada. I found this rock as a kid in Korea in a high mountain area. I was always wondering what this thing was. Its very heavy, and also very hard. A knife wouldn't even make a mark on it. The green crystal looking area on the second picture is very smooth, While the first picture still has a greenish color but rough. As you can see on the third picture it is slightly transparent. Size is about 20 centimeters, ruler looks off due to perspective. If needed I'll try to break it with hammer, but I dont want to risk it at the moment.
The physical appearance of your rock texture, color, luster, and fracture suggests that it is likely to be nephrite jade. A nephrite deposit can be found in Chuncheon City, South Korea. Oksanga jade mine is reported to be still an active source for white nephrite jade. Other jade occurrences most likely exist in South Korea and dark olive green is a common color for nephrite.