No default operator type for indexes for numeric fields???

I have the following table structure:

+----------+-----------------------------+-------+
|  Field   |              Type           | Length|
+----------+-----------------------------+-------+
| perioada | int2 not null               |     2 |
| gestiune | int2 not null               |     2 |
| produs   | int4 not null               |     4 |
| pret     | numeric not null            | 30.2  |
| initial  | numeric not null default 0  | 25.3  |
| intrari  | numeric not null default 0  | 25.3  |
| iesiri   | numeric not null default 0  | 25.3  |
+----------+-----------------------------+-------+ 

Trying to create an index like

create unique index stocuri_pkey on stocuri
(perioada,gestiune,produs,pret);

I got the following error:

ERROR:  Can't find a default operator class for type 1700.

How should I create an index using a numeric field?

Constantin Teodorescu
FLEX Consulting Braila, ROMANIA