Ruby is a pink to blood red gemstone, a variety of the mineral corundum (aluminium oxide). The common red color is caused mainly by the element chromium. Its name comes from ruber, Latin for red. Other varieties of gem-quality corundum are called sapphires. It is considered one of the four precious stones, together with the sapphire, the emerald and the diamond. Improvements used include color alteration, improving transparency by dissolving rutile inclusions, healing of fractures (cracks) or even completely filling them. Prices of rubies are primarily determined by color (the brightest and best "red" called Pigeon Blood Red, command a huge premium over other rubies of similar quality). After color follows clarity: similar to diamonds, a clear stone will command a premium, but a ruby without any needle-like rutile inclusions will indicate the stone has been treated one way or another. Cut and carat (size) will also determine the price. Rubies have a hardness of 9.0 on the Mohs scale of mineral hardness. Among the natural gems only diamond is harder, with a Mohs 10.0 by definition.
 

bài 2: theo mình nên dùng 3 ý cơ bản như sau:
1. $RANDOM: mỗi lần sẽ in 1 số ngẫu nhiên từ 0-32767: vd: number=$RANDOM
2. dùng command let: let "number %= 10" --> sinh ngẫu nhiên từ 0-9
3. dùng 2 vòng lặp for hoặc while để in thành ma trận. 

#!/bin/bash
for ((x=0; x<10; x++)) 
	do 
		y=0
		until [ "$y" = "9" ]
			do
				number=$RANDOM
				let "number %=10"
				echo -n "$number "
				y=$((y + 1))
			done
		number=$RANDOM
		let "number %=10"
		echo $number
	done
exit 0

number=$RANDOM
		let "number %=10"
		echo $number

#!/bin/bash
for ((x=0; x<10; x++)) 
	do 
		for ((y=0; y<10; y++)) 
			do
				number=$RANDOM
				let "number %=10"
				echo -n "$number "
			done
		echo ""
	done
exit 0