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I'll solve this problem by analyzing the pattern in the numbers within the diamonds.

First, I need to understand the relationship between the numbers at the corners and the number in the center of each diamond. Let me examine the first two diamonds to find a pattern.

First Diamond:
- Top: 1
- Right: 5
- Bottom: 3
- Left: 5
- Center: 6

Second Diamond:
- Top: 3
- Right: 7
- Bottom: 4
- Left: 8
- Center: 8

Let me look for a mathematical relationship. One common pattern is that the center number might be the sum or average of the corner numbers, or perhaps some combination.

For the first diamond: 1 + 5 + 3 + 5 = 14, which is not 6.
Maybe it's the sum of opposite corners? 1 + 3 = 4, 5 + 5 = 10. Not obviously related to 6.
What if it's the average? 14/4 = 3.5, not 6.

Let me try another approach. Maybe the center is the sum of two specific corners minus something.

Another idea: maybe it's the sum of the top and bottom minus the difference of left and right? That seems too complex.

Let me try a simpler pattern: what if the center is the sum of the top and bottom numbers?
First diamond: 1 + 3 = 4, not 6.
Second diamond: 3 + 4 = 7, not 8.

What if it's the sum of all four corners divided by something? 14/2.33... = 6? That doesn't seem clean.

Let me try: top + bottom + (left - right) or something similar.

Wait, let me try: (top + bottom) + (left - right)
First diamond: (1 + 3) + (5 - 5) = 4 + 0 = 4, not 6.
Second diamond: (3 + 4) + (8 - 7) = 7 + 1 = 8. That works for the second diamond!

Let me check if this pattern works for the first diamond with a different arrangement.
What if it's (top + bottom) + (right - left)?
First diamond: (1 + 3) + (5 - 5) = 4 + 0 = 4, still not 6.

Another idea: maybe it's the average of the sum of opposite corners?
First diamond: (1+3)/2 = 2, (5+5)/2 = 5, then 2+5=7, not 6.

Let me try: top + left = 1 + 5 = 6. That's the center! Let me check the second diamond: top + left = 3 + 8 = 11, not 8.

What about top + right? First diamond: 1 + 5 = 6. Second diamond: 3 + 7 = 10, not 8.

Bottom + left? First: 3 + 5 = 8, not 6. Second: 4 + 8 = 12, not 8.

Bottom + right? First: 3 + 5 = 8, not 6. Second: 4 + 7 = 11, not 8.

Let me try a different approach. What if the center is the sum of the two smaller numbers or something like that?

First diamond numbers: 1, 3, 5, 5. The two smallest are 1 and 3, sum is 4, not 6.
The two largest are 5 and 5, sum is 10, not 6.

Another idea: maybe it's the product of two numbers divided by something? That seems unlikely given the small numbers.

Let me try: (top * bottom) / something. 1*3=3, not 6.

Wait, let me look at it differently. What if the center is the sum of the top and bottom, plus the difference between left and right, but absolute value?

First diamond: (1 + 3) + |5 - 5| = 4 + 0 = 4, not 6.

What if it's (left + right) - (top + bottom)? First: (5+5)-(1+3)=10-4=6. That works!
Second diamond: (8+7)-(3+4)=15-7=8. That also works!

Perfect! The pattern is: Center = (Left + Right) - (Top + Bottom)

Now let's apply this to the third diamond:
- Top: 1
- Right: 9
- Bottom: 5
- Left: 4
- Center: ?

Center = (Left + Right) - (Top + Bottom) = (4 + 9) - (1 + 5) = 13 - 6 = 7

Therefore, the missing number is 7.