Matrix.Diagonal($\{x_1,x_2,...,x_n\}$) takes a list of $n$ numbers and returns an $n \times n$ diagonal matrix with the $x_i$ as entries.
If the option “Userdata” is set to true (default is false) the output is returned as a pointer rather than a table of tables (compare Matrix.ToUserdata()).
xlist = {17,3,-4*I, sqrt(6)}
M = Matrix.Diagonal(xlist)
print(M)
{ { 17 , 0 , 0 , 0 } ,
{ 0 , 3 , 0 , 0 } ,
{ 0 , 0 , (-0 - 4 I) , 0 } ,
{ 0 , 0 , 0 , 2.4494897427832 } }